BIP20,21: add Superseded-By and Replaces headers (#1984)

bip-0020.mediawiki

@@ -7,6 +7,7 @@
   Type: Specification
   Assigned: 2011-01-10
   License: BSD-2-Clause
+  Proposed-Replacement: 21
 </pre>
 
 BIP 0020 is based off an earlier document by Nils Schneider. '''And has been replaced by BIP 0021'''

bip-0021.mediawiki

@@ -7,6 +7,7 @@
   Status: Closed
   Type: Specification
   Assigned: 2012-01-29
+  Replaces: 20
   Proposed-Replacement: 321
 </pre>