Add generic OAuth fields to the config docs

[dariusk]

I did the best I could and I might have gotten the proxy fields wrong because I haven't used them myself. I tried to infer what they did by reading the code.

In addition to generic OAuth, I added disable_password_auth under the [app] section.

Partially addresses #30, and reflects the "scope" field I added in writefreely/writefreely#402.

[mentos1386]

@dariusk thanks for providing this. Helped me a lot setting up generic oauth. One thing that is missing is [oauth.generic] to know where to provide values to.

[dariusk]

Thanks @mentos1386, I updated it to include the section header.

[thebaer]

Thanks for documenting this, @dariusk! And thanks for the input, @mentos1386. This looks good to me!