contrib: Add README for pgp keys

contrib/gitian-keys/README.md

@@ -0,0 +1,16 @@
+PGP keys
+========
+
+This folder contains the public keys of developers and active contributors.
+
+The keys are mainly used to sign git commits or the build results of gitian
+builds.
+
+You can import the keys into gpg as follows. Also, make sure to fetch the
+latest version from the key server to see if any key was revoked in the
+meantime.
+
+```sh
+gpg --import ./*.pgp
+gpg --refresh-keys
+```