Add a markdownlint pre-commit hook

This adds a pre-commit hook to link Markdown files and fixes all
reported findings.

README.md

@@ -15,7 +15,6 @@ There are several ways to contact us and find more information:
 - Gitea (development info, bug tracker): https://gitea.wildfiregames.com/
 - IRC: #0ad on irc.quakenet.org
 
-
 ## Running precompiled binaries on Windows
 
 A precompiled, ready-to-play development version of the game is available from
@@ -27,33 +26,28 @@ In a checkout of the `nightly-build` SVN repository, open the "binaries\system"
 - To launch the game: Run pyrogenesis.exe
 - To launch the map editor: Run Atlas.bat or "pyrogenesis.exe -editor"
 
-
 ## Compiling the game from source code
 
 The instructions for compiling the game on Windows, Linux and OS X are at
 [BuildInstructions](https://gitea.wildfiregames.com/0ad/0ad/wiki/BuildInstructions).
 
-
 ## Reporting problems
 
 Bugs should be reported on Gitea. For information on reporting problems
 and finding logs, see [ReportingErrors](https://gitea.wildfiregames.com/0ad/0ad/wiki/ReportingErrors).
 
-
 ## Contributing Code
 
 If you want to help out programming for the game, have a look at
 [GettingStartedProgrammers](https://gitea.wildfiregames.com/0ad/0ad/wiki/GettingStartedProgrammers)
 or contact us on #0ad-dev on irc.quakenet.org
 
-
 ## Contributing Artwork
 
 If you want to make artwork for the game, have a look at
 [For Artists](https://gitea.wildfiregames.com/0ad/0ad/wiki#for-artists)
 or visit [the forums](https://www.wildfiregames.com/forum).
 
-
 ## Translating
 
 You can help translating the game at https://www.transifex.com/projects/p/0ad