Add a header to the response to indicate the expiry time of the api key

[bgreen1005]

The documentation says that the api key is valid for 60 minutes. Can we add a header to the response to indicate exactly when the api key will expire? I have a use case where I might have multiple clients calling the authorisation api to get a token. If client A gets a token and it's valid for 1 hour, then 15 minutes later the token is retrieved from client B, client B will assume that it's also valid for 1 hour. This would be much simpler if we had a timestamp header.

I'm happy to make the changes myself if that's easier. If I wanted to do so, would it be a case of forking from master or one of the other branches?

[bgreen1005]

Realised that this is the repo for documentation, not for the api.