Clarify scope of project

[agrimm]

Quite a few issues or offers of patches seem to be related to people wanting to do "developer access token" requests, as opposed to "client access token" requests.

Although you mention the word "client" in the name of the repo and as one of the class names, it could easily be mistaken as merely the opposite of "server" rather than the opposite of "developer". Would it be possible to add a sentence or two to the README just clarifying that it doesn't handle "developer access token" requests? Also, if there's a separate project that handles "developer access token" requests, a link to that project would be good.