Merge bitcoin#24788: doc: Add gpg key import instructions for Windows

1075820 doc: Add gpg key import instructions for Windows (Dave Scotese)

Pull request description:

  This is a single commit to replace the three commits from bitcoin#23916

  I propose this change so that Windows users can more easily import signers' keys.

ACKs for top commit:
  sipsorcery:
    tACK 1075820.

Tree-SHA512: 7d4ec77ce10f751748c49f1453fa8baf0976b15af4f87dc27f4e2715ad73fbd7dc1f07fcf3e660d63a6b9eb895a5e4105774613d39a2328f73b92d9e6cff4ebd

Author: fanquake

contrib/builder-keys/README.md

@@ -19,9 +19,15 @@ gpg --refresh-keys
 To fetch keys of builders and active developers, feed the list of fingerprints
 of the primary keys into gpg:
 
+On \*NIX:
 ```sh
 while read fingerprint keyholder_name; do gpg --keyserver hkps://keys.openpgp.org --recv-keys ${fingerprint}; done < ./keys.txt
 ```
 
+On Windows (requires Gpg4win >= 4.0.0):
+```
+FOR /F "tokens=1" %i IN (keys.txt) DO gpg --keyserver hkps://keys.openpgp.org --recv-keys %i
+```
+
 Add your key to the list if you provided Guix attestations for two major or
 minor releases of Bitcoin Core.