Create README.md

[styfle]

This adds a README.md file with introduction text copied from

https://developers.google.com/caja/

Fixes #1973 and related to #1987

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If your company signed a CLA, they designated a Point of Contact who decides which employees are authorized to participate. You may need to contact the Point of Contact for your company and ask to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the project maintainer to go/cla#troubleshoot.
• In order to pass this check, please resolve this problem and have the pull request author add another comment and the bot will run again.

[kpreid]

I like this. Just two things:

• Like the bot says, you need to sign the Google CLA.
• Please modify the commit message to mention where the text was copied from, since it was.

[googlebot]

CLAs look good, thanks!

[styfle]

Thanks, I signed it!

I also updated the commit message.