Add PGP external validations (keybase, keyservers)

[emanuelb]

To have more assurance of accurate/trusted information of the PGP key, adding below fields will be useful:

1. keybase page (for example, search for "Security Team" in keybase)
2. mention the availability of the PGP key on key-servers.

[EdOverflow]

This was included in the Encryption: directive. Please refer to Section 2.4: https://www.ietf.org/id/draft-foudil-securitytxt-00.txt.