ADD: Require Schnorr BIP for Taproot

Per https://github.com/bitcoin/bips/blob/master/bip-0001.mediawiki:

"BIPs may have a Requires header, indicating the BIP numbers that this BIP depends on"

bip-taproot.mediawiki

@@ -9,6 +9,7 @@
   Type: Standards Track
   Created:
   License: BSD-3-Clause
+  Requires: BIP-Schnorr
 </pre>
 
 ==Introduction==