Add alt texts to badges

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Add readthedocs badge

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Fix some links in README.
Merge pull request #462 from Trundle/fix_links

Fix some links in README.
Improve mentions of 3rd party extensions

Add more extensions, and mention all extensions in an appropriate
context throughout the manual.
Drop support for Web Mode [#464]
Simplify predicate for eruby checker
Fix SASS/SCSS test cases and error patterns
Work around D Mode void-function error

See https://github.com/Emacs-D-Mode-Maintainers/Emacs-D-Mode/issues/23
[Fix #460] Extract column information from DMD
Merge branch 'dmd-columns'

* dmd-columns:
  [Fix #460] Extract column information from DMD
  Work around D Mode void-function error
Update Texinfo manual
Reference Contributor Guide in README

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Fix filename

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Remove reference to non-existing file

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[Fix #461] Drop Projectile integration

Gets in the way of ancestor-based config file locating
Skip test if required modes are missing
Remove last traces of s.el in the test suite

Regression from fca2c54
Add a Coq syntax checker
Provision Coq on test systems