Add Contributing docs to README

[jsma]

Something similar to https://github.com/torchbox/wagtailmedia#contributing would be very helpful.

I'm not very familiar with tox (beyond blindly following Contributing docs), but I was unable to run specific tests e.g.:

 tox -e python3.11-django4.2-wagtail5.0 tests.test.test_blocks.TestBlocks
usage: tox [-h] [--colored {yes,no}] [-v | -q] [--exit-and-dump-after seconds] [-c file] [--workdir dir] [--root dir] [--runner {virtualenv}] [--version] [--no-provision [REQ_JSON]] [--no-recreate-provision] [-r] [-x OVERRIDE]
           {run,r,run-parallel,p,depends,de,list,l,devenv,d,config,c,quickstart,q,exec,e,legacy,le} ...
tox: error: unrecognized arguments: tests.test.test_blocks.TestBlocks
hint: if you tried to pass arguments to a command use -- to separate them from tox ones

Thanks to @nickmoreton for getting the existing tests in place!

[zerolab]

@jsma try tox run -e python3.11-django4.2-wagtail5.0 -- tests.test.test_blocks.TestBlocks or tox -e python3.11-django4.2-wagtail5.0 -- tests.test.test_blocks.TestBlocks

[jsma]

Ah, interesting. I see I should have read that last sentence of the output more closely but only just now realized the difference between what I had invoked and the wagtailmedia example I was borrowing from (tox -e py310-dj41-wagtail41 tests.test_views.TestMediaChooserUploadView) was that I was calling the run sub-command. Thanks for the pointer!

[zerolab]

@jsma I believe your PR #56 addressed this, so it can be closed, no?