update README.md

[kiridaruma]

Now, this library supporting JWK in #273 but I could not find it in document.

sorry for my poor English.
if added example don't make sence, please tell me and will fix it.

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[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).

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• 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).
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[kiridaruma]

@googlebot I signed it!

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[nsundberg]

Just want to point out a typo here that could cause confusion.

Suggested change

	$convertedKeys = JWT::parseKeySet($keys);
	$convertedKeys = JWK::parseKeySet($keys);

[bshaffer]

The readme now has an example of using JWKs! So I believe this is now a duplicate of #307