Add local testing instructions to the contributors' guidelines

[ferozsalam]

No description provided.

[nsano-rururu]

I think you should run sudo make clean before running make test-docker. When I added the python library to requirements.txt, it couldn't be installed successfully.

Also, if you look at docker ps -a, after make test-docker is finished, the container will accumulate more and more with the container stopped, so you need to delete it regularly.

[ferozsalam]

@nsano-rururu I have updated the PR to include sudo make clean, thanks.

Regarding the containers that are left behind, I think that can be fixed by changing docker-compose --project-name elastalert run tox to docker-compose --project-name elastalert run --rm tox in the Makefile. I'm just confirming now, but if it all looks OK I'll put in a separate diff for that in a moment.