Include Gitter link everywhere.

CONTRIBUTING.md

@@ -56,6 +56,8 @@ Contact
 
 *   Website -- http://mosra.cz/blog/magnum.php
 *   GitHub -- https://github.com/mosra/magnum
+*   Gitter -- https://gitter.im/mosra/magnum
+*   IRC -- join `#magnum-engine` channel on freenode
 *   Google Groups -- https://groups.google.com/forum/#!forum/magnum-engine
 *   Twitter -- https://twitter.com/czmosra
 *   E-mail -- mosra@centrum.cz

README.md

@@ -175,6 +175,7 @@ idea? Feel free to visit my website or contact me at:
 
 *   Website -- http://mosra.cz/blog/magnum.php
 *   GitHub -- https://github.com/mosra/magnum
+*   Gitter -- https://gitter.im/mosra/magnum
 *   IRC -- join `#magnum-engine` channel on freenode
 *   Google Groups -- https://groups.google.com/forum/#!forum/magnum-engine
 *   Twitter -- https://twitter.com/czmosra

doc/mainpage.dox

@@ -115,6 +115,7 @@ Feel free to get more information or contact the author at:
 
 -   Website -- http://mosra.cz/blog/magnum.php
 -   GitHub -- https://github.com/mosra/magnum
+-   Gitter -- https://gitter.im/mosra/magnum
 -   IRC -- join `#magnum-engine` channel on freenode
 -   Google Groups -- https://groups.google.com/forum/#!forum/magnum-engine
 -   Twitter -- https://twitter.com/czmosra