CONTRIBUTING.rst: Add instructions to update requirements files

      ForceCommand BST_PROFILE=artifact-receive cd /tmp && bst-artifact-receive --pull-url https://example.com/ /home/artifacts/artifacts

The MANIFEST.in and setup.py

----------------------------

When adding a dependency to BuildStream, it's important to update the setup.py accordingly.

Managing data files

-------------------

When adding data files which need to be discovered at runtime by BuildStream, update setup.py accordingly.

When adding data files for the purpose of docs or tests, or anything that is not covered by

creating a tarball which contains everything we want it to include::

  ./setup.py sdist

Updating BuildStream's Python dependencies

------------------------------------------

BuildStream's Python dependencies are listed in multiple

`requirements files <https://pip.readthedocs.io/en/latest/reference/pip_install/#requirements-file-format>`

present in the ``requirements`` directory.

All ``.txt`` files in this directory are generated from the corresponding

``.in`` file, and each ``.in`` file represents a set of dependencies. For

example, ``requirements.in`` contains all runtime dependencies of BuildStream.

``requirements.txt`` is generated from it, and contains pinned versions of all

runtime dependencies (including transitive dependencies) of BuildStream.

When adding a new dependency to BuildStream, or updating existing dependencies,

it is important to update the appropriate requirements file accordingly. After

changing the ``.in`` file, run the following to update the matching ``.txt``

file::

   make -C requirements