Clarify inline the CL acronym definition

[aleris]

Maybe not everyone knows what CL stands for, this puts the definition when first used
in the text, to avoid looking up in other places. There is a definition in README.md
however that is not linked and not an obvious way to look into for it.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[aleris]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[ninachen]

Hi there,
"CL" appears enough times in the content -- and is not a commonly used acronym outside of Google -- that I'm thinking we could change the content more broadly to be more understandable regardless of what page you start on.

You can follow along on #22

[benmoss]

I came here to suggest the same thing! Thanks for this!