[added] update README regarding onAfterOpen and Jest

[postrad]

Changes proposed:

• Providing an additional section in the README related to the post v3.14.1 change to the behavior of the onAfterOpen prop and how it can break Jest tests related to it.

I am not sure if this is the best way to communicate this change, but I can speak from experience that this was an unexpectedly disruptive one in my organization, and hopefully this can spare others the time it took me to discover the source of a number of failing tests when I made the minor update.

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 84.737% when pulling 8c0f219 on postrad:docs/on-after-open-jest into b33923a on reactjs:master.