bip174: document PSBT 0 handling of type > 0xFD

bip-0174.mediawiki

@@ -819,6 +819,9 @@ If an updater is updating a PSBT and needs to add a field that is only available
 New fields should first be proposed on the bitcoin-dev mailing list.
 If a field requires significant description as to its usage, it should be accompanied by a separate BIP.
 The field must be added to the field listing tables in the Specification section.
+Although some PSBT version 0 implementations encode types as uint8_t rather than compact size,
+it is still safe to add >0xFD fields to PSBT 0, because these old parsers ignore
+unknown fields, and <keytype> is prefixed by its length. 
 
 ===Procedure For New Versions===