Explain what should happen for revocation

wiki/src/doc/about/openpgp_keys/signing_key_revocation.mdwn

@@ -197,6 +197,11 @@ revoking the Tails signing key ourselves anymore, then this person
 should write to the mailing list explaining why she thinks that the
 signing key needs to be revoked.
 
+People on the list who are also convinced that the signing key should be
+revoked share their shares until they have 3 different shares. Then they
+can assemble the revocation certificate and publish it to revoke the
+signing key.
+
 Yes, there is no mathematically proven algorithm for this and here is
 where your judgement as a human being is needed. The description of the
 very bad event should be checked or backed by enough people to be