doc(contributor): Expand the contributor guide to be more exhaustive …

…and reference the just file

docs/contributor_guide.rst

@@ -1,7 +1,30 @@
 Contributor's Guide
 ===================
 
+This page is dedicated to the technical information that you may find useful when contributing code or documentation to the UP repository.
+For information on the contribution process, please have a look to the `corresponding page <https://github.com/aiplan4eu/unified-planning/blob/master/CONTRIBUTING.md>`_.
 
+Common commands (Justfile)
+--------------------------
+
+A list of common commands and recipes are available in a `justfile <https://github.com/aiplan4eu/unified-planning/blob/master/justfile>`_.
+You can use it to find out common commands (e.g. running tests, reformatting the code) or use it directly with the `just <https://github.com/casey/just>`_ command line tool.
+
+Whenever you see a command ``just RECIPE_NAME`` in this page, it means the command is available as a just recipe.
+You can run it directly with ``just`` or check the ``justfile`` for the actual commands to run.
+
+Checklist
+---------
+
+Before sending a pull-request, you should check the following conditions:
+
+ - your code is properly formatted with ``black`` (``just format``)
+ - your code passes the linter check (``just check-mypy``)
+ - your code passes all existing unit tests
+ - you have added tests to ensure that your change is and remains valid
+ - you have updated the documentation to reflect your changes
+
+Whenever possible these are automatically check in CI.
 
 Documentation
 -------------
@@ -13,8 +36,7 @@ It consists of a collection of reStructuredText documents and python notebooks t
 Building the documentation
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-
-The documentation can be built locally like so::
+The documentation can be built locally like so (``just build-doc``)::
 
     cd docs/  # enter the documentation folder
     pip install -r requirements.txt  # install documentation toolchain
@@ -30,11 +52,21 @@ The reference API part of the documentation is built automatically by the `docs/
 The script contains the list of classes that will appear in the documentation.
 If you contribute a new *user-facing* class, the list of classes should be updated to make it appear in the API reference.
 
+Ensuring documentation consistency
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+As much as possible, we strive to make example code testable to ensure that it remains valid when updating the library.
+This notably include all code in ``docs/code_snippets`` as well as all notebooks in ``docs/notebooks``.
 
+If your notebook has some code that should *not* be run in CI, the corresponding cell should be tagged with the ``remove_from_CI`` flag.
 
+Protobuf bindings
+-----------------
 
+The UP provides protobuf bindings that can be used export the problems modeled with the UP in a protobuf format.
+The schema for this format are available in the `unified_planning/grpc/unified_planning.proto <https://github.com/aiplan4eu/unified-planning/blob/master/unified_planning/grpc/unified_planning.proto>`_ file.
 
-Issue Tracking
---------------
+When updating them you should ensure that:
 
-Issue tracking is done in GitHub issues: https://github.com/aiplan4eu/unified-planning/issues
+ - the schema remains backward compatible (no removal, no change in the fields indices)
+ - you have regenerated the python bindings available in the repository (``just gen-protobuf``)