Add discovery for Indieauth

[pstuifzand]

I just noticed Home Assistant supports authorization with Indieauth. I found that while it supports Indieauth that the discovery part of the spec isn't implemented. This would make it easier to automatically get a token from Home Assistant.

<link rel="authorization_endpoint" href="https://your-instance.com/auth/authorize" />
<link rel="token_endpoint" href="https://your-instance.com/auth/token" />

I'm not sure where to start to implement this, but I would like to contribute the change, if someone could point me in the right direction :)