bip174: add global xpub field

Author: achow101

bip-0174.mediawiki

@@ -118,6 +118,12 @@ The currently defined global types are as follows:
 *** <tt>{transaction}</tt>
 ** Note: Every PSBT must have a field with this type.
 
+* Type: Extended Public Key <tt>PSBT_GLOBAL_XPUB = 0x01</tt>
+** Key: The type followed by the 78 byte serialized extended public key as defined by BIP 32. Extended public keys are those that can be used to derive public keys used in the inputs and outputs of this transaction. It should be the public key at the highest hardened derivation index so that the unhardened child keys used in the transaction can be derived.
+*** <tt>{0x01}|{xpub}</tt>
+** Value: The master key fingerprint as defined by BIP 32 concatenated with the derivation path of the public key. The derivation path is represented as 32 bit unsigned integer indexes concatenated with each other. The number of 32 bit unsigned integer indexes must match the depth provided in the extended public key.
+*** <tt>{master key fingerprint}|{32-bit int}|...|{32-bit int}</tt>
+
 The currently defined per-input types are defined as follows:
 
 * Type: Non-Witness UTXO <tt>PSBT_IN_NON_WITNESS_UTXO = 0x00</tt>