Improve the contribution docs, add a test-sqlite make command and fix tox.ini errors

[orf]

I noticed a few issues while bootstrapping this project for contributions. I ran into a tox error with some tox.ini values:

tox.tox_env.errors.Fail: pass_env values cannot contain whitespace, use comma to have multiple values in a single line, invalid values found 'PYTEST_ADDOPTS TERM TEST_DB_USER TEST_DB_PASSWORD TEST_DB_HOST'

make testenv has also been removed, and I added a make test-sqlite command that bootstraps everything and runs the tests on the latest stable version of Django.