Update CONTRIBUTING.md to forewarn about the CLA requirement.

Per the suggestion from @schmod in #2871 (comment), this adds a note that contributors will be expected to sign a Contributor License Agreement (CLA) when contributing to the Apollo Server project.

CONTRIBUTING.md

@@ -66,7 +66,7 @@ For significant changes to a repository, it’s important to settle on a design
 2.  **Reach consensus.** Some contributors and community members should reach an agreement that this feature or bug is important, and that someone should work on implementing or fixing it.
 3.  **Agree on intended behavior.** On the issue, reach an agreement about the desired behavior. In the case of a bug fix, it should be clear what it means for the bug to be fixed, and in the case of a feature, it should be clear what it will be like for developers to use the new feature.
 4.  **Agree on implementation plan.** Write a plan for how this feature or bug fix should be implemented. What modules need to be added or rewritten? Should this be one pull request or multiple incremental improvements? Who is going to do each part?
-5.  **Submit PR.** In the case where multiple dependent patches need to be made to implement the change, only submit one at a time. Otherwise, the others might get stale while the first is reviewed and merged. Make sure to avoid “while we’re here” type changes - if something isn’t relevant to the improvement at hand, it should be in a separate PR; this especially includes code style changes of unrelated code.
+5.  **Submit PR.** In the case where multiple dependent patches need to be made to implement the change, only submit one at a time. Otherwise, the others might get stale while the first is reviewed and merged. Make sure to avoid “while we’re here” type changes - if something isn’t relevant to the improvement at hand, it should be in a separate PR; this especially includes code style changes of unrelated code. Keep in mind that, upon opening a PR, if the _Contributor License Agreement (CLA)_ hasn't been previously signed, it will be requested that you sign it. The CLA [can be previewed here](https://contribute.meteor.com).
 6.  **Review.** At least one core contributor should sign off on the change before it’s merged. Look at the “code review” section below to learn about factors are important in the code review. If you want to expedite the code being merged, try to review your own code first!
 7.  **Merge and release!**