Added css class div.warning, used for the known issue docs quickfix on M1.
Added release-notes directory and notes for 0.5.0-M2.
Fix docs build: re-add eclipse.include.md

- docs/use/guide/quicksart/eclipse.include.md had been accidentally
  deleted, which caused the jekyll build to fail. This commit
  reinstates that file.
Adds link to release-notes (in download.html)
Release notes list formatting fix.
Easier getting started path on download page.
Also one small optimization for release notes download location on snapshot vs milestone docs build.
Moved eclipse.include.md closer to usage (ide.md) in file hierarchy, to prevent future accidental deletion and repeat of docs build failure.
Merge pull request #582 from jdtoy/master

Docs improvements following M2 release.