Add links where non-members can make patent licensing committments to CONTRIBUTING.md

[zacknewman]

Currently CONTRIBUTING.md states:

To contribute, you must either participate in the relevant W3C Working Group or make a non-member patent licensing commitment

however nowhere in that document are there links where one can do so. This makes it difficult for non-members to make contributions (e.g., PRs) because they may not know where to go so that they can make such non-member patent licensing commitments.

[simoneonofri]

Updated here #2131