Add MELPA badges to README
Re-order badges

Travis state is least interesting for casual users, so let's move it to
the end.

Thanks, @purcell
Add flycheck-add-mode function

Lets libraries add new modes to existing syntax checkers
Rebuild Texinfo manual
[Fix #507] Add OpenMP support to GCC
Rebuild Texinfo manual
Update MELPA links
Update some outdated links
Add release notes
Bump version
Rebuild Texinfo manual
Merge branch 'release-0.21'

* release-0.21:
  Rebuild Texinfo manual
  Bump version
  Add release notes
Bump version on master

[ci skip]
Fix issue references in changelog

[ci skip]
[Fix #508] Parse notes from shellcheck