Add Contributions section to README to point at the CLA.
Add Contributions section to README to point at the CLA.

NichoZhang/lint,golang/lint,tmc/lint,lutzky/lint,zimmski/lint,JekaMas/hint,elgris/hint,kovetskiy/lint,gorcz/lint,darkliquid/lintci,hellosuper/gohint