Add instructions for setting up a dev environment to README

[joeroe]

i.e. a dockerless server setup. Something like:

1. Install external dependencies: ruby, bundler, yarn, postgresql
2. Set up postgresql: create a xronos user with CREATEDB privileges
3. Fill in env_variables.env and move it to ./.env
4. Run bundle && yarn to install dependencies
5. Run bin/rails db:setup to set up and seed the database
6. Start a dev server with bin/dev*

* If there's an error about missing foreman, restart your terminal.

[joeroe]

Having now done this for the third or fourth time, I think I've figured out a smooth set of instructions (above). @MartinHinz could you check whether there's anything stupid/unnecessary? Then I will add to the README so I don't have to puzzle it out again next time!

[MartinHinz]

At first glance, this looks correct and complete to me. If this is how it worked for you for the third time, it should also be correct ;-) I would have said, not such a high priority, but if it is going to be helpful for yourself, then go ahead!

[joeroe]

Would need to be revised if/when we move from webpacker #83.

[joeroe]

Updated the instructions, which work quite smoothly for me.