Backend protocol wanted features

Hi,

I've collected "wanted features" page (see [1], [2]).
A dedicated file in pgjdbc seems fine for me. If you know better
location, waive a hand.

If you remember more, or if you want your name with "+1" to appear
there, comment on the PR or file a PR as that one is merged in.

[1]: https://github.com/pgjdbc/pgjdbc/pull/478
[2]: https://github.com/pgjdbc/pgjdbc/blob/protocol_wanted_features/backend_protocol_v4_wanted_features.md

PS. As always, it would be nice if you proofread the file as I am
having troubles with mastering English.

Vladimir