Add a simple developer documentation for new contributors

[salty-ivy]

          @salty-ivy running the tests is not that well documented in the readme, here is how I did it.

Assuming you are inside a virtualenv

# Install wagtail-ab-testing in editable mode along with its testing dependencies
pip install -e ".[testing]"

# Run the tests
python testmanage.py test

Originally posted by @Stormheg in #53 (comment)

Cc: @Stormheg

We should add small developer section in readme.

that should at least documents how to install the repo locally and runtests.

this would include this repo to the scope of new upcoming contributors.