add info in the readme how one may run tests

[matkoniecz]

currently, at least for me, it is not obvious

[dahlia]

Sorry for the late response. We use tox or detox to run tests. Tox purposes to run a test suite for multiple Python interpreters (multiple versions and multiple VMs like CPython and PyPy). Detox is a parallelized version of tox. If a tox is installed in your system it's easy to run tests:

tox