Document application configuration options

[grahamalama]

In #204, we added an environment variable SENTRY_TRACES_SAMPLE_RATE to control the volume of transactions sent to Sentry (docs).

When discussing this change, we found that the documentation of this environment variable was lacking, and that we might also want to document other important environment variables, especially ones that aren't completely obvious from the name alone, e.g.

APP_RELOAD
APP_DEBUG
MAX_RETRIES

[bsieber-mozilla]

Add a link to the environment.py file on README. (Add context: use as all caps..)
Add additional Documentation to environment.py