Add info about which signing keys will be used for published artifacts.

[yogurtearl]

Add info about which signing keys will be used for published artifacts.

For security purposes, it would be great if you were able to publish the gpg public keys that are "valid" for use when verifying signing artifacts uploaded to maven central.

This allows for "out of band" verification of the expected signing key.

Some examples of other libs publishing their signing keys:

https://square.github.io/okhttp/security/security/#verifying-artifacts

https://github.com/eclipse/jetty.project/blob/jetty-10.0.x/KEYS.txt
https://downloads.apache.org/commons/KEYS
https://downloads.apache.org/logging/KEYS

Looks like com.google.j2objc:j2objc-annotations:2.8 was signed with this key:

https://keyserver.ubuntu.com/pks/lookup?search=EE92349AD86DE446&fingerprint=on&op=index

[yogurtearl]

Dup of #2099