Add link to Google Groups to documentation.

CONTRIBUTING.md

@@ -53,8 +53,9 @@ Code contribution
 Contact
 -------
 
-*   Website - http://mosra.cz/blog/magnum.php
-*   GitHub - https://github.com/mosra/magnum
-*   Twitter - https://twitter.com/czmosra
-*   E-mail - mosra@centrum.cz
-*   Jabber - mosra@jabbim.cz
+*   Website -- http://mosra.cz/blog/magnum.php
+*   GitHub -- https://github.com/mosra/magnum
+*   Google Groups -- https://groups.google.com/forum/#!forum/magnum-engine
+*   Twitter -- https://twitter.com/czmosra
+*   E-mail -- mosra@centrum.cz
+*   Jabber -- mosra@jabbim.cz

README.md

@@ -163,11 +163,12 @@ CONTACT
 Want to learn more about the library? Found a bug or want to tell me an awesome
 idea? Feel free to visit my website or contact me at:
 
-*   Website - http://mosra.cz/blog/magnum.php
-*   GitHub - https://github.com/mosra/magnum
-*   Twitter - https://twitter.com/czmosra
-*   E-mail - mosra@centrum.cz
-*   Jabber - mosra@jabbim.cz
+*   Website -- http://mosra.cz/blog/magnum.php
+*   GitHub -- https://github.com/mosra/magnum
+*   Google Groups -- https://groups.google.com/forum/#!forum/magnum-engine
+*   Twitter -- https://twitter.com/czmosra
+*   E-mail -- mosra@centrum.cz
+*   Jabber -- mosra@jabbim.cz
 
 LICENSE
 =======

doc/mainpage.dox

@@ -109,11 +109,12 @@ make the library as consistent and maintainable as possible.
 
 Feel free to get more information or contact the author at:
 
-- Website - http://mosra.cz/blog/magnum.php
-- GitHub - https://github.com/mosra/magnum
-- Twitter - https://twitter.com/czmosra
-- E-mail - mosra@centrum.cz
-- Jabber - mosra@jabbim.cz
+-   Website -- http://mosra.cz/blog/magnum.php
+-   GitHub -- https://github.com/mosra/magnum
+-   Google Groups -- https://groups.google.com/forum/#!forum/magnum-engine
+-   Twitter -- https://twitter.com/czmosra
+-   E-mail -- mosra@centrum.cz
+-   Jabber -- mosra@jabbim.cz
 
 @section mainpage-license License