Add examples on how to sign a token with key pair keys

[rickyseezy]

It would be great to have a bit of documentation on how to sign a token using private/public pair keys.

I've found an example here : http://www.inanzzz.com/index.php/post/kdl9/creating-and-validating-a-jwt-rsa-token-in-golang

[diyfr]

https://github.com/golang-jwt/jwt/issues/61#issue-959067303

[mfridman]

We have a docs repository, we should probably chip away at adding copy/pasteable examples. Contributions welcome.

https://github.com/golang-jwt/jwt-docs