Added support for `exp` to README. Borrowed much of the verbiage from [PyJWT's README](https://github.com/progrium/pyjwt/blob/90beaa320b6ca0a4765ace46b1355d9b2ebadf77/README.md).
Added support for `exp` to README.
Borrowed much of the verbiage from [PyJWT's README](https://github.com/progrium/pyjwt/blob/90beaa320b6ca0a4765ace46b1355d9b2ebadf77/README.md).

progrium/ruby-jwt,larrylv/ruby-jwt,ernie/ruby-jwt,bkeepers/ruby-jwt,iamjarvo/ruby-jwt,ab320012/ruby-jwt,excpt/ruby-jwt,skippy/ruby-jwt,danoph/ruby-jwt,jwt/ruby-jwt,yahooguntu/ruby-jwt,aj-michael/ruby-jwt