Add small contributing document with information about codestyle checking

Add argument to set list of source files to check

This is necessary, in case you want to use docker run in pretxncommit
Mercurial hook. Unfortunately for some reason if hg running in
container can't find just created commit defined by $HG_NODE. And it
throws error 'unknown revision'.

Here is example for hgrc.

[hooks]
pretxncommit.linter = hg status -m -n -a -n -I '**.py' --change $HG_NODE > files.lst && docker run --volume=$PWD:/beremiz --workdir="/beremiz" --volume=$PWD/../CanFestival-3:/CanFestival-3 --memory=1g --entrypoint=/beremiz/tests/tools/check_source.sh skvorl/beremiz-requirements --files-to-check files.lst

check_source.sh: handle case if there are no files to check

Fix linter errors