Dev PGP keys: revoke Pieter Wuille's key

- Also add some hints for future maintainers about key revocation to
  README.md per discussion in PR #1120

README.md

@@ -870,3 +870,15 @@ Below the YAML front matter, enter the content of the post in Markdown
 format.  Images should be placed in `img/blog/free` if they are
 MIT-licensed or `img/blog/nonfree` if they have a more restrictive
 copyright license.
+
+### Developer PGP keys
+
+The site hosts the PGP keys for several Bitcoin Core contributors. Here
+are some notes about updating those keys based on previous experience:
+
+1. If a key is revoked, update the key with the revocation immediately.
+   Anyone with commit access to the site repository may do this without
+   prior review, but they should post the commit ID to an open issue or
+   PR so other people can review it. After the revoked key is uploaded,
+   discussion about verifying/adding a replacement key may continue at a
+   slower pace.