Add a few documents for VTEC decoding and handling reference

doc/vtec-schema.txt

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+WFO 	3 Char 	This is the three character NWS Office/Center identifier. For CONUS locations, this is the 4 character ID dropping the first K. For non-CONUS sites, this is the identifier dropping the P.
+ISSUED 	12 Char 	This timestamp represents the start time of the event. When an event's lifecycle begins, this issued value can be updated as the NWS issues updates. The value presented represents the last known state of the event start time.
+EXPIRED 	12 Char 	Similiar to the ISSUED column above, this represents the products event end time. Again, this value is updated as the event lifecycle happens with updates made by the NWS.
+INIT_ISS 	12 Char 	This is timestamp of the NWS Text Product that started the event. This timestamp is important for products like Winter Storm Watch, which have a begin time a number of days/hours into the future, but are typically considered to be in effect at the time of the text product issuace. Yeah, this is where the headaches start. This timestamp can also be used to form a canonical URL back to the IEM to fetch the raw NWS Text for this event. It is not updated during the event's lifecycle.
+INIT_EXP 	12 Char 	Similiar to INIT_ISS above, this is the expiration of the event denoted with the first issuance of the event. It is not updated during the event's lifecycle.
+PHENOM or TYPE 	2 Char 	This is the two character NWS identifier used to denote the VTEC event type. For example, TO for Tornado and SV for Severe Thunderstorm. A lookup table of these codes exists here.
+SIG 	1 Char 	This is the one character NWS identifier used to denote the VTEC significance. The same link above for PHENOM has a lookup table for these.
+GTYPE 	1 Char 	Either P for polygon or C for county/zone/parish. The shapefiles you download could contain both so-called storm-based (polygon) events and traditional county/zone based events.
+ETN 	Int 	The VTEC event identifier. A tracking number that should be unique for this event, but sometimes it is not. Yes, more headaches. Note that the uniqueness is not based on the combination of a UGC code, but the issuance center and a continuous spatial region for the event.
+STATUS 	3 Char 	The VTEC status code denoting the state the event is during its life cycle. This is purely based on any updates the event got and not some logic on the IEM's end denoting if the event is in the past or not.
+NWS_UGC 	6 Char 	For county,zone,parish warnings GTYPE=C, the Universal Geographic Code that the NWS uses. Sadly, this is not exactly FIPS.
+AREA_KM2 	Number 	The IEM computed area of this event, this area computation is done in Albers (EPSG:9311).
+UPDATED 	12 Char 	The timestamp when this event's lifecycle was last updated by the NWS.
+HV_NWSLI 	5 Char 	For events that have H-VTEC (Hydro VTEC), this is the five character NWS Location Identifier.
+HV_SEV 	1 Char 	For events that have H-VTEC (Hydro VTEC), this is the one character flood severity at issuance.
+HV_CAUSE 	2 Char 	For events that have H-VTEC (Hydro VTEC), this is the two character cause of the flood.
+HV_REC 	2 Char 	For events that have H-VTEC (Hydro VTEC), this is the code denoting if a record crest is expected at issuance.
+EMERGENC 	Boolean 	Based on unofficial IEM logic, is this event an "Emergency" at any point during its life cycle.
+POLY_BEG 	12 Char 	In the case of polygons (GTYPE=P) the UTC timestamp that the polygon is initially valid for.
+POLY_END 	12 Char 	In the case of polygons (GTYPE=P) the UTC timestamp that the polygon expires at.
+HAILTAG 	Number 	The IBW hail size tag (inches). This is only included with the (GTYPE=P) entries as there is a 1 to 1 association between the tags and the polygons. If you do not include SVS updates, it is just the issuance tag.
+WINDTAG 	Number 	The IBW wine gust tag (MPH). See HAILTAG.
+TORNTAG 	16 Char 	The IBW tornado tag. See HAILTAG.
+DAMAGTAG 	16 Char 	The IBW damage tag. See HAILTAG.
+PROD_ID 	36 Char 	Issuance text. IEM identifier used to uniquely (99% of the time) identify NWS Text Products. The value can be passed to https://mesonet.agron.iastate.edu/p.php?pid=PROD_ID for a website viewer or against the IEM API service https://mesonet.agron.iastate.edu/api/1/nwstext/PROD_ID.

doc/vtec-test.txt

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+932 
+WFUS53 KDMX 212103
+TORDMX
+IAC015-079-187-212145-
+/O.NEW.KDMX.TO.W.0045.240521T2103Z-240521T2145Z/
+
+BULLETIN - EAS ACTIVATION REQUESTED
+Tornado Warning
+National Weather Service Des Moines IA
+403 PM CDT Tue May 21 2024
+
+The National Weather Service in Des Moines  has issued a
+
+* Tornado Warning for...
+  Southeastern Webster County in central Iowa...
+  Northwestern Boone County in central Iowa...
+  Southwestern Hamilton County in central Iowa...
+
+* Until 445 PM CDT.
+
+* At 403 PM CDT, a severe thunderstorm capable of producing a tornado
+  was located over Pilot Mound, or 8 miles northwest of Boone, moving
+  northeast at 30 mph.
+
+  HAZARD...Tornado and quarter size hail.
+
+  SOURCE...Radar indicated rotation.
+
+  IMPACT...Flying debris will be dangerous to those caught without 
+           shelter. Mobile homes will be damaged or destroyed. 
+           Damage to roofs, windows, and vehicles will occur.  Tree 
+           damage is likely.
+
+* This dangerous storm will be near...
+  Stratford around 410 PM CDT.
+  Stanhope around 420 PM CDT.
+  Kamrar around 435 PM CDT.
+
+Other locations impacted by this tornadic thunderstorm include
+Ellsworth, Stanhope, Webster City Municipal Airport, Pilot Mound,
+Stratford, Kamrar, and Fraser.
+
+This includes Interstate 35 between mile markers 133 and 137.
+ 
+PRECAUTIONARY/PREPAREDNESS ACTIONS...
+
+TAKE COVER NOW! Move to a basement or an interior room on the lowest
+floor of a sturdy building. Avoid windows. If you are outdoors, in a
+mobile home, or in a vehicle, move to the closest substantial shelter
+and protect yourself from flying debris.
+
+&&
+
+LAT...LON 4204 9397 4215 9409 4251 9384 4234 9351
+TIME...MOT...LOC 2103Z 221DEG 26KT 4216 9397 
+
+TORNADO...RADAR INDICATED
+MAX HAIL SIZE...1.00 IN
+
+$$
+
+Donavon