Improve documentation on configuring plugin

[psrpinto]

Docs are not too clear on how to configure the keys. We should mention specifically that they should be set in wp-config.php, and could also mention using wp-cli to set it.

Docs are also not too clear on how to set oidc_clients. It's suggested to use functions.php, which is not ideal because it's tied to the theme. We should probably suggest using a plugin (or probably even an mu-plugin) to configure the oidc_clients.

[jeherve]

+1 on this. I personally got stuck when defining the clients. I have no idea where to find the necessary info to create a new client (in my case, I would like to use my site to log in to OpenStreetMap).

[biguenique]

+1. I Agree. It's pretty hard guessing out of the blue what strings should be changed to what when registering clients in the current state of the README. That makes a lot of pretty unclear parameters at once to play with. Just a little context or examples would be very useful here.