add release notes based on release 3.3

Name suggestion was from Helmut K. C. Tessarek
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 07e7ebf5)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

RELEASE_NOTES

+
+              ┌───────────────────────────────────────┐
+              │ RELEASE NOTES for FFmpeg 3.4 "Cantor" │
+              └───────────────────────────────────────┘
+
+   The FFmpeg Project proudly presents FFmpeg 3.4 "Cantor", about 6
+   months after the release of FFmpeg 3.3.
+
+   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.