Add 'Getting Started' guide to README (#33)

Co-authored-by: Victor Stinner <[email protected]>

Author: Erlend Egeberg Aasland

README.rst

@@ -10,10 +10,44 @@ The ``pythoncapi-compat`` project can be used to write a C extension supporting
 a wide range of Python versions with a single code base. It is made of the
 ``pythoncapi_compat.h`` header file and the ``upgrade_pythoncapi.py`` script.
 
-* Homepage: `GitHub pythoncapi-compat project
-  <https://github.com/python/pythoncapi-compat>`_.
-* `Documentation
-  <https://pythoncapi-compat.readthedocs.io/en/latest/>`_
+``upgrade_pythoncapi.py`` requires Python 3.6 or newer.
+
+See the `documentation at ReadTheDocs
+<https://pythoncapi-compat.readthedocs.io/en/latest/>`_
+for more details.
+
+Getting started
+===============
+
+To upgrade a specific file::
+
+    python3 upgrade_pythoncapi.py module.c
+
+To upgrade all C/C++ files in a directory::
+
+    python3 upgrade_pythoncapi.py src/
+
+Select operations
+-----------------
+
+To only replace ``op->ob_type`` with ``Py_TYPE(op)``, select the ``Py_TYPE``
+operation with::
+
+    python3 upgrade_pythoncapi.py -o Py_TYPE module.c
+
+Or the opposite, to apply all operations but leave ``op->ob_type`` unchanged,
+deselect the ``Py_TYPE`` operation with::
+
+    python3 upgrade_pythoncapi.py -o all,-Py_TYPE module.c
+
+Download pythoncapi_compat.h
+----------------------------
+
+If you want to ``pythoncapi_compat.h`` to your code base, use the
+``upgrade_pythoncapi.py`` tool to fetch it::
+
+    python3 upgrade_pythoncapi.py --download PATH
+
 
 This project is distributed under the `Zero Clause BSD (0BSD) license
 <https://opensource.org/licenses/0BSD>`_ and is covered by the `PSF Code of