Update README.md

Author: Michael Davis

README.md

@@ -41,6 +41,15 @@ JSON Web Signatures (JWS) are used to digitally sign a JSON encoded object and r
 
 JSON Web Tokens (JWT) are a JWS with a set of reserved claims to be used in a standardized manner.
 
+#### JWT Reserved Claims
+
+Claim | Name | Format | Usage
+------|------|--------|------
+'exp' | Expiration | int | The time after which the token is invalid.
+'nbf' | Not Before | int | The time before which the token is invalid.
+'iss' | Issuer | str | The principal that issued the JWT.
+'aud' | Audience | str or list(str) | The recipient that the JWT is intended for.
+'iat' | Issued At | int | The time at which the JWT was issued.
 
 ## Algorithms