docs: add note that GCC 4.8 is needed to build gluon

The used nodejs 4.4.5 needs at least GCC 4.8 to be build
https://github.com/nodejs/node/blob/v4.x/BUILDING.md



Signed-off-by: Matthias Fritzsche <matthias@chemnitz.freifunk.net>
[Matthias Schiffer: moved information to the first section]