doc: Add 1.5 release notes to release-notes.rst

The only copy of the release notes for releases prior to 1.6 is on
the zephyrproject.org/downloads site.  Preserving the release notes
history in the git repo would be a good thing.  Here are the notes
for 1.5 (more to come).

Also added an auto table-of-contents header to make it easier to
navigate.

Change-Id: Ibd8b5a8d3816f7f5c376216adc2d52bb9467214d
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>