No usage doc explanation

[rdegges]

The usage docs for this library are pretty blank. It currently just shows a code snippet with no explanation of what's going on. It'd be cool to get a read guide that explains what JWTs are, what this library does in more detail, and how to work with JWTs using this library.

[bretterer]

@rdegges Does this resolve the issue here? https://github.com/okta/okta-jwt-verifier-php/tree/update_readme

[rdegges]

Great!!