README: add a very short development setup guide

README.md

@@ -6,3 +6,48 @@ core library for swh's modules:
 - hash computations
 - serialization
 - logging mechanism
+- database connection
+- http-based RPC client/server
+
+Development
+-----------
+
+We strongly recommend you to use a [virtualenv][1] if you want to run tests or
+hack the code.
+
+To set up your development environment:
+
+```
+(swh) user@host:~/swh-environment/swh-core$ pip install -e .[testing]
+```
+
+This will install every Python package needed to run this package's tests.
+
+Unit tests can be executed using [pytest](https://docs.pytest.org).
+
+```
+(swh) user@host:~/swh-environment/swh-core$ pytest
+============================== test session starts ==============================
+platform linux -- Python 3.7.3, pytest-3.10.1, py-1.8.0, pluggy-0.12.0
+hypothesis profile 'default' -> database=DirectoryBasedExampleDatabase('/home/ddouard/src/swh-environment/swh-core/.hypothesis/examples')
+rootdir: /home/ddouard/src/swh-environment/swh-core, inifile: pytest.ini
+plugins: requests-mock-1.6.0, hypothesis-4.26.4, celery-4.3.0, postgresql-1.4.1
+collected 89 items
+
+swh/core/api/tests/test_api.py ..                                         [  2%]
+swh/core/api/tests/test_async.py ....                                     [  6%]
+swh/core/api/tests/test_serializers.py .....                              [ 12%]
+swh/core/db/tests/test_db.py ....                                         [ 16%]
+swh/core/tests/test_cli.py ......                                         [ 23%]
+swh/core/tests/test_config.py ..............                              [ 39%]
+swh/core/tests/test_statsd.py ........................................... [ 87%]
+....                                                                      [ 92%]
+swh/core/tests/test_utils.py .......                                      [100%]
+===================== 89 passed, 9 warnings in 6.94 seconds =====================
+```
+
+Please read the [developer setup manual][5] for more information on how to hack
+on Software Heritage.
+
+[2]: https://docs.softwareheritage.org/devel/developer-setup.html
+[1]: https://virtualenv.pypa.io