Update CONTRIBUTING.md

Language corrections (as well as spelling Linnovate right!). I would very much prefer you include a copyright assignment clause. Here you are only licensing the contribution. Some communitied do not like copyright assignment, but I personnaly, for this piece of code find it simpler.  At least for larger contributions and contributions made by companies. See http://dreamsongs.com/ihe/IHE-110.html for an example clause..
s/contributre/contribute