Update CONTRIBUTING.md link
Update CONTRIBUTING.md link

The `CONTRIBUTING.md` was moved to `.github`  in change commit 96d7d1c271afce3d91b82f620d89e060ac5f7876 but the `README` wasn't updated
Correcting this to make life better for anyone going to the `README` in order to find that document
imolorhe/altair,imolorhe/altair,imolorhe/altair,imolorhe/altair