Added testId prop and usage to documentation

[callmenick]

Changes proposed:

• Added testId prop existence and usage to documentation

Upgrade Path (for changed or removed APIs):

Acceptance Checklist:

• The commit message follows the guidelines in CONTRIBUTING.md.
• Documentation (README.md) and examples have been updated as needed.
• If this is a code change, a spec testing the functionality has been added.
• If the commit message has [changed] or [removed], there is an upgrade path above.

[coveralls]

Coverage remained the same at 87.135% when pulling d5a30f6 on callmenick:patch-1 into 8efaa37 on reactjs:master.

[diasbruno]

Thanks, @callmenick. Can you also check if we have a test for this, please?

[callmenick]

No problem @diasbruno. I do indeed see that it's tested here:

https://github.com/reactjs/react-modal/blob/master/specs/Modal.spec.js#L434