tests: py.test removal from install requirements

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
docs: removal of pull/push mentions

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
setup: initial release of `CONTRIBUTING`

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>