docs: refine instructions for proxy setting

[thejohncrafter]

This is formally a PR but it's more of an issue.

I was having trouble deploying Hydra behind a reverse proxy, I wanted the url to look like hydra.example.com.
I didn't need a prefix path, so I didn't set up X-Request-Base -- a fatal mistake. Hydra didn't set up its links correctly, and I was systematically redirected to 127.0.0.1:_.

I corrected the docs so that someone who wants to do something like me won't make the same mistake :

Serving behind reverse proxy
----------------------------

[...]

You also need to configure your reverse proxy to pass `X-Request-Base`
to hydra, with the same value as `base_uri`.

I hope this is the correct and idiomatic fix, but I'm not fluent in perl/Catalyst, feel free to dismiss this PR 🚀