Add a section on how to run under a sub-dir

[archaeogeek]

This adds the start of a section on how to run geohealthcheck under a sub-dir. It's taken me a while to figure out how to do this in my setup without changing any core code, using nginx and gunicorn, and the end solution was so simple that I thought it was worth contributing. Unfortunately I don't know how it would be done using any other webserver! I do wonder whether I should also contribute a section on how to run geohealthcheck using nginx and gunicorn anyway!

[justb4]

Thanks @archaeogeek, possibly you may add some extra wording: this issue has popped up in several deployment scenarios, like with Docker. There we usually refer to "running GHC with a sub-path" (i.s.o. subdir) or "running GHC on a subpath behind a proxy server". Like you (and we painstakingly) found out, the "secret" is using the SCRIPT_NAME environment variable at GHC's startup. This var is interpreted by Flask such that e.g. Jinja2 url_for() directives render correct paths.