Added easier ccache workaround for Fedora

Just a minor update to the docs as I've been looking at the mess that is ccache on Fedora 20 (seems to break most embedded compilers!)

Save for deleting /etc/profile.d/ccache.sh and ccache.csh, setting ```AVR_TOOLS_DIR=/usr``` seems like the easiest way to use the regular compilers and bypass ccache on Fedora.

Current instructions show how to use ccache for the compilers but not linkers etc; so they still have value so have left them in.