[doc] Added basic contribution guide

doc/contributing.rst

+.. _contributing:
+
+Hacking CAISAR
+==============
+.. index:: single: Driver; driver
+
+CAISAR project structure
+************************
+
+The root of the repository has the following folders:
+
+* ``bin/`` contains utils for instanciating python-based prover for CAISAR.
+* ``ci/caisar-public/`` contains automated scripts for
+  replicating CAISAR repository on partner forges.
+* ``config/`` contains various utilities for CAISAR to
+  interface with external provers. In particular, driver definitions are located in ``config/drivers/``.
+* ``doc/`` contains all material related to documentation generation.
+* ``docker/`` contains utils to build CAISAR docker image for
+  release.
+* ``examples/`` contains examples of WhyML files
+  representing known properties that CAISAR can verify.
+* ``lib/`` contains OCaml library files that are primarly
+  written to interface with CAISAR, but can be used in other
+  projects as well.
+* ``licenses/`` contains license files.
+* ``src/`` contains most of the OCaml source of CAISAR.
+* ``stdlib/`` contains WhyML files used to define
+  theories in our interpreted language.
+* ``tests/`` are non-regression tests written in `dune cram test syntax <https://dune.readthedocs.io/en/stable/tests.html>`_.
+
+
+Setting up a development environment
+************************************
+
+TODO
+
+.. index:: Prover;prover
+
+Opening a merge request
+***********************
+
+We gladly accept merge requests (MR) on our `public forge <https://git.frama-c.com/pub/caisar/-/merge_requests>`_.
+Some possible topics for opening a MR are:
+
+* support for a new prover
+* enhancing the support for an existing prover
+* bug fixes