Update references to Gentoo

Gentoo has officially merged the ebuild into portage as of:
https://github.com/gentoo/gentoo/pull/9925

Let's update the readme and get rid of the `contrib/gentoo-bees`
directory, so we have no potentially outdated information in the future.

Signed-off-by: Kai Krakow <kai@kaishome.de>

README.md

@@ -367,16 +367,25 @@ Bees is available in Arch Linux AUR. Install with:
 
 `$ pacaur -S bees-git`
 
-Gentoo ebuild
--------------
+Gentoo package
+--------------
 
-Bees is available as a Gentoo ebuild. Just copy `bees-9999.ebuild` from
-`contrib/gentoo` including the `files` subdirectory to your local
-overlay category `sys-fs`.
+Bees is officially available in Gentoo Portage. Just emerge a stable
+version:
 
-You can copy the ebuild to match a Bees version number, and it will
-build that tagged version. It is partly supported since v0.5,
-previous versions won't work.
+`$ emerge --ask bees`
+
+or build a live version from git master:
+
+`$ emerge --ask =bees-9999`
+
+You can opt-out of building the support tools with
+
+`USE="-tools" emerge ...`
+
+If you want to start hacking on bees and contribute changes, just emerge
+the live version which automatically pulls in all required development
+packages.
 
 Build from source
 -----------------
@@ -400,7 +409,10 @@ Packaging
 ---------
 
 See 'Dependencies' below. Package maintainers can pick ideas for building and
-configuring the source package from the Gentoo ebuild in `contrib/gentoo`.
+configuring the source package from the Gentoo ebuild:
+
+https://github.com/gentoo/gentoo/tree/master/sys-fs/bees
+
 You can configure some build options by creating a file `localconf` and
 adjust settings for your distribution environment there.