update gitian-keys readme (fixes gpg fetching)

subset.pool.sks-keyservers.net has no keys, @cevap extended it
to patch from:

1. Ubuntu Keyserver
2. Debian Keyserver
3. MIT Keyserver
4. SKS keyservers used by OpenPGP

Fixes ioncoincore#17
Fixes ioncoincore#18

contrib/gitian-keys/README.md

@@ -20,6 +20,13 @@ To fetch keys of Gitian builders and active developers, feed the list of
 fingerprints of the primary keys into gpg:
 
 ```sh
+# Ubuntu keyserver
+while read fingerprint keyholder_name; do gpg --keyserver hkp://keyserver.ubuntu.com --recv-keys ${fingerprint}; done < ./keys.txt
+# Debian keyring
+while read fingerprint keyholder_name; do gpg --keyserver hkp://keyring.debian.org --recv-keys ${fingerprint}; done < ./keys.txt
+# MIT keyserver
+while read fingerprint keyholder_name; do gpg --keyserver hkp://pgp.mit.edu--recv-keys ${fingerprint}; done < ./keys.txt
+# SKS keyservers used by OpenPGP
 while read fingerprint keyholder_name; do gpg --keyserver hkp://subset.pool.sks-keyservers.net --recv-keys ${fingerprint}; done < ./keys.txt
 ```