Added EMF_changelog.txt & is_high_prio=yes to emf_quarterly_pulse

POLICY ON THE EMF CHANGELOG:

Log everything! Do not spare internal (user-invisible) or even WIP
development changes. When I make an EMF release (someday), it will
be my job to distill the changelog into a user-relevant change list
to go along with the release announcement. However, do note that
EMF_changelog.txt is in the root of the actual mod directory (like
PB), so it is distributed with the mod; users will be able to go
over the detailed, purely sequential changelog (which means later
entries may render earlier entries null-- e.g., beta bugfixes of
features introduced during that same beta). This dual audience is
intended. Let them watch us work.

More importantly, let us be quite aware, in quick to digest terms,
of each other's changes throughout EMF and keep a solid log for
historical awareness.