Summary:
Add quick start document for new developers.
Add developer documentation for C++ code style.
Add git and arc workflow document.
This is the first step in moving parts of the documentation related to
development from Phabricator wiki to git tracked markdown files. Most of
the changes here overhaul the text from the wiki, to make it more
friendly for new developers.
Markdown uses the Github flavored extensions, and should easily
be converted to any other document using `pandoc`.
Reviewers: buda, msantl, mculinovic, mtomic
Reviewed By: mtomic
Subscribers: mislav.bradac
Differential Revision: https://phabricator.memgraph.io/D1031
