Document monkeysign

Originally created by @sajolida on #11240 (Redmine)

I managed to send monkeysign emails doing:

torsocks monkeysign --user=sajolida@pimienta.org --smtp=mssatgg2rwa4aytk.onion:587 --smtpuser=sajolida@pimienta.org 0x66c8c2d7c5aa446d

So it works and we should document this briefly as an “advanced topics”.

Feature Branch: emmapeel:docs/11240-monkeysign

Edited May 15, 2020 by sajolida

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