LetsEncrypt support is not yet on master.

Writing clarifies thinking and leaves behind guidance for future
maintainers.  Design descriptions shouldn't be required, though,
especially for trivial modules.

Users expect role authors to make decisions.  The role author is
supposed to be the expert who knows what choices to make on behalf of
the user.

I probably got this wrong, but I'm putting a stake in the ground based
on work I've done on the Roundcube module and adding Let's Encrypt
support to the common module.

added details about the license and the supported distributions.

Reflects the move to the `sovereign` GitHub organization.