Update README to reflect Issue #6 ?

[hallyn]

Issue#6 points to PR#7 which is merged. However, that PR by itself doesn't fix the problem, it only allows the problem to be fixed.

The README still shows adding a key to the usersession keychain, and reading it from that. The keyring.Add should be updated to reflect this comment: https://github.com/jsipprell/keyctl/issues/6#issuecomment-921896695 . That is, add the key to the session keyring, set the perms, link to usersession and unlink from session.

Without that, subsequent key.Get() will fail. With that, it works.