Doc++, updated credits.

CREDITS.md

@@ -17,5 +17,9 @@ Contributors to Magnum library
     port improvements
 *   Stefan Wasilewski ([@smw](https://github.com/smw)) -- Mac OS X port
     improvements
+*   Konstantinos Chatzilygeroudis ([@costashatz](https://github.com/costashatz)) --
+    Debian package
+*   Olga Turanksaya ([@olga-python](https://github.com/olga-python)) -- Gentoo
+    ebuild
 
 Big thanks to everyone involved!

doc/building.dox

@@ -315,6 +315,10 @@ project root:
 
     makepkg -p package/archlinux/<file>
 
+@subsection building-packages-gentoo Gentoo ebuilds
+
+Gentoo Git ebuild is available in `package/gentoo` directory.
+
 @subsection building-packages-deb DEB packages
 
 There is also `package/debian/` directory with all files needed for building