release notes (based on release/3.0)

Better release notes are welcome
write better ones or do not complain later!
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

RELEASE_NOTES

+
+              ┌────────────────────────────────────────┐
+              │ RELEASE NOTES for FFmpeg 3.1 "Laplace" │
+              └────────────────────────────────────────┘
+
+   The FFmpeg Project proudly presents FFmpeg 3.1 "Laplace", about 4
+   months after the release of FFmpeg 3.0.
+
+   A complete Changelog is available at the root of the project, and the
+   complete Git history on http://source.ffmpeg.org.
+
+   We hope you will like this release as much as we enjoyed working on it, and
+   as usual, if you have any questions about it, or any FFmpeg related topic,
+   feel free to join us on the #ffmpeg IRC channel (on irc.freenode.net) or ask
+   on the mailing-lists.