Update release notes for 1.14

Pinging @larkost and really everyone else. This is almost entirely from the closed tickets on the 1.14 milestone excluding bugs that seemed like they never made it into releases, but there are likely things missed and/or things that should be excluded. I tried to mention the bits of backward incompatibility in the "Compatibility" block.
This says "you should rebuild your index before updating to the next version of RethinkDB".  "indexes" should be plural, but more importantly we want people to rebuild the outdated indexes **after** they update to get the new behavior.  (Also, we call it "upgrading" rather than "updating" above.  It's common to use "update" to refer to a small fix and "upgrade" to refer to a big version change with new features.  We should probably use "upgrade" for this release.)