Introduce contributors faq

Related to T3119

docs/faq.rst

+.. _faq:
+
+Frequently Asked Questions
+==========================
+
+How can I compute my SWHID?
+---------------------------
+
+We propose a cli to ease such computation ``swh identify``...
+
+how should I document the code I contribute to SWH?
+---------------------------------------------------
+
+Any new feature should include documentation in the form of comments and/or docstrings.
+Ideally, they should also be documented in plain English in the repository's `docs/`
+folder if relevant to a single package, or in the main `swh-docs` repository if it is a
+transversal feature.
+
+What are the minimum system requirements (hardware/software) to run SWH locally?
+--------------------------------------------------------------------------------
+
+See the `developer setup documentation <https://docs.softwareheritage.org/devel/developer-setup.html#developer-setup>`__
+

docs/index.rst

-.. _swh-docs:
 
 
 Software Heritage - Development Documentation
 =============================================
 
-
 Getting started
 ---------------
 
@@ -46,6 +44,8 @@ Tutorials
 * :doc:`/tutorials/issue-debugging-monitoring`
 * :ref:`Listing the content of your favorite forge <lister-tutorial>`
   and :ref:`running a lister in Docker <run-lister-tutorial>`
+* :ref:`faq`
+
 
 Roadmap
 -------
@@ -200,6 +200,7 @@ Indices and tables
    architecture/index
    contributing/index
    tutorials/index
+   faq
    roadmap/roadmap-2021.rst
    infrastructure/index
    swh.auth <swh-auth/index>