Add readme

[matthew-white]

getodk has a few different XLSForm-related repositories (and would have one more with pyxform-http), so it might be useful to add a short readme here explicitly connecting this repository to the XLSForm Online web app.

[yanokwa]

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[matthew-white]

Ideas for things that could be documented in a readme:

• How to run the app
• List of environment variables. I needed to set DJANGO_SECRET_KEY and DJANGO_TMP_HOME.
• How are static assets served? Do I need to configure something? I get 404s for bootstrap.min.css and jquery.min.js.

[yanokwa]

Just so I don't forget, the rough instructions are...

pip3 install -r requirements.txt
export DJANGO_SECRET_KEY=SECRET
export DJANGO_TMP_HOME=/tmp
python3 manage.py migrate
python3 manage.py collectstatic --noinput
python3 manage.py runserver

We should probably package this is Docker.