Add instructions in README for customizing database name, password, username with ENV

[krschacht]

Ah, yes, all rails apps support the DATABASE_URL env, both for development and production. The production configs (render, fly, heroku) already set this. But this should definitely be in the README! The format for this variable should be postgres://username:password@localhost/hostedgpt_development

Also explain that you can customize just the DB with HOSTEDGPT_DEV_DB

Originally posted by @krschacht in #453 (reply in thread)

[draknor]

Hmmm - how do I link this issue to the PR I opened #456 ? Sorry - still learning the Github workflows!

[krschacht]

@draknor In the PR description you can mention an issue, e.g, "Fixes #454" and then it links them up.