Mention the new official Twitter hashtag.

README.md

@@ -133,7 +133,8 @@ possible.
 -   Gitter community chat — https://gitter.im/mosra/magnum
 -   E-mail — info@magnum.graphics
 -   Google Groups mailing list — magnum-engine@googlegroups.com ([archive](https://groups.google.com/forum/#!forum/magnum-engine))
--   Author's personal Twitter — https://twitter.com/czmosra
+-   Twitter — https://twitter.com/czmosra and the
+    [#MagnumEngine](https://twitter.com/hashtag/MagnumEngine) hashtag
 
 See also the Magnum Project [Contact & Support page](https://magnum.graphics/contact/)
 for further information.

doc/mainpage.dox

@@ -67,8 +67,10 @@ keep the library as consistent and maintainable as possible.
 -   GitHub project page --- https://github.com/mosra/magnum
 -   Gitter community chat --- https://gitter.im/mosra/magnum
 -   E-mail --- info@magnum.graphics
--   Google Groups mailing list --- magnum-engine@googlegroups.com ([archive](https://groups.google.com/forum/#!forum/magnum-engine))
+-   Google Groups mailing list --- magnum-engine@googlegroups.com
--   Author's personal Twitter --- https://twitter.com/czmosra
+    ([archive](https://groups.google.com/forum/#!forum/magnum-engine))
+-   Twitter --- https://twitter.com/czmosra and the
+    [\#MagnumEngine](https://twitter.com/hashtag/MagnumEngine) hashtag
 
 See also the [Contact & Support page](https://magnum.graphics/contact/) on the
 project website for further information.