thunderbird integration incompatible with armor keyword in gpg.conf

[snqk]

See context here: https://thunderbird.topicbox.com/groups/e2ee/T98c38d698d7fc1aa

TL;DR: when setting up decryption for emails in the thunderbird client, the armor keyword caused decryption attempts to fail as TB expects a dearmored output.

I'd propose adding a warning for anyone who stumbles onto this issue.

[drduh]

That would be good information to include in the Email section, along with the throw-keyids option notice for Mailvelope.

Can you send a PR?

[snqk]

@drduh thanks, this is ready for a review