contributing: rewrite code style recommendation (#2842)
contributing: rewrite code style recommendation (#2842)

As per https://github.com/coreos/docs/pull/1142, coreos golang code
style has been removed from cores/docs. This commit rewrites the
code style recommendation to link to the one maintained by
the go community.
lander2k2/tectonic-installer,kyoto/tectonic-installer,cpanato/tectonic-installer,lander2k2/tectonic-installer,cpanato/tectonic-installer,derekhiggins/installer,kalmog/tectonic-installer,kalmog/tectonic-installer,squat/tectonic-installer,kyoto/tectonic-installer,cpanato/tectonic-installer,derekhiggins/installer,squat/tectonic-installer,lander2k2/tectonic-installer,cpanato/tectonic-installer,lander2k2/tectonic-installer,kyoto/tectonic-installer,squat/tectonic-installer,kyoto/tectonic-installer,kyoto/tectonic-installer,squat/tectonic-installer,kalmog/tectonic-installer