Add CLA link (mochajs#3020) [ci skip]

* Add link to CLA in README

* Add CLA link to CONTRIBUTING document

.github/CONTRIBUTING.md

@@ -62,7 +62,7 @@ Follow these steps to get going.  If you are having trouble, don't be afraid to
 1.  Navigate to the source repository.  You should see a notification about your recent changes in your fork's branch, with a button to create a pull request.  Click it.
 1.  Describe your changes in detail here.  Once you're satisfied, submit the form.
   - *PRO TIP*: If you've used a multi-line commit message, Github will pre-fill the PR's description with it.
-1.  If you have not signed our Contributor License Agreement, a friendly robot will prompt you to do so.  A CLA (electronic) signature is **required** for all contributions of code to Mocha.
+1.  If you have not signed our Contributor License Agreement, a friendly robot will prompt you to do so.  A [CLA](https://cla.js.foundation/mochajs/mocha) (electronic) signature is **required** for all contributions of code to Mocha.
 1.  CI will run against your changes.
   - If the changes fail the checks, you will need to address those before merging.
   - You don't need to make a new PR to make changes.  Instead, commit on top of your changes, and push these to your fork's branch.  The PR will be updated, and CI will re-run.

README.md

@@ -19,7 +19,7 @@
 2.  Review current open pull requests
   - We need individuals familiar with Mocha's codebase. Got questions?  Ask them in [our chat room](https://gitter.im/mochajs/mocha).
   - Pull requests **must** have supporting tests.  The only exceptions are pure cosmetic or non-functional changes.
-  - Pull request contributors must sign the CLA.
+  - Pull request contributors must sign [the CLA](https://cla.js.foundation/mochajs/mocha).
 3. Close old, inactive issues and pull requests
   - ~~A bot should do this.  We need a bot. Got a bot?~~ We now use GitHub's own [probot-stale](https://www.npmjs.com/package/probot-stale).
 4. Triage issues