doc/developer: revise mailing list section

The Developer Documentation had instructions to
subscribe to the ffmpeg-cvslog email list. But that is
no longer accurate. For the purposes in this section --
review of patches, discussion of development issues --
ffmpeg_devel is the appropriate email list. Some developers
may want to monitor ffmpeg-cvslog, but it is not mandatory.

This is v3 of this doc, based on discussion in thread
<https://ffmpeg.org/pipermail/ffmpeg-devel/2017-November/220528.html>
and in response to docs Maintainer comments in
<https://ffmpeg.org/pipermail/ffmpeg-devel/2017-December/221596.html>.

1. In doc/developer.texi, add a new section about
ffmpeg-devel, based on existing text from ffmpeg-cvslog
section regarding discussion of patches and of
development issues. Reflect wording from discussion at
<https://ffmpeg.org/pipermail/ffmpeg-devel/2017-November/221199.html>
but with copy-editing to make wording more concise.

2. In doc/developer.texi, rewrite the ffmpeg-cvslog section
to match the current usage of ffmpeg-cvslog. Some
developers choose to follow this list, but it is not
mandatory.

There are a lot of improvements possible to the
Developer Documentation page, beyond this refactoring.
However, making those improvements is a much bigger
and more difficult task.  This change is "low hanging
fruit".

Signed-off-by: Jim DeLaHunt <from.ffmpeg-dev@jdlh.com>
Signed-off-by: Timothy Gu <timothygu99@gmail.com>

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