Update CONTRIBUTING.md
Update CONTRIBUTING.md

add links to conventions and guidelines for contributing
Anwaario/Nick-Tallguy.github.io,jmarlena/learnosm-ux-staging-site,Anwaario/Nick-Tallguy.github.io,hotosm/learnosm,jmarlena/learnosm-ux-staging-site,feyeandal/test,althio/learnosm,althio/learnosm,furaidochikin/learnosm,Nick-Tallguy/Nick-Tallguy.github.io,feyeandal/test,jmarlena/learnosm-ux-staging-site,feyeandal/test,jmarlena/learnosm-ux-staging-site,Nick-Tallguy/Nick-Tallguy.github.io,furaidochikin/learnosm,Anwaario/Nick-Tallguy.github.io,hotosm/learnosm,Anwaario/Nick-Tallguy.github.io,furaidochikin/learnosm,hotosm/learnosm,jmarlena/learnosm,althio/learnosm,jmarlena/learnosm,feyeandal/test,jmarlena/learnosm,althio/learnosm,Nick-Tallguy/Nick-Tallguy.github.io,Nick-Tallguy/Nick-Tallguy.github.io,hotosm/learnosm,jmarlena/learnosm,Nick-Tallguy/Nick-Tallguy.github.io,furaidochikin/learnosm