Add a first stab at a README.

Closes https://github.com/TypedDjango/django-stubs/issues/1.

README.md

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+[![Gitter chat](https://badges.gitter.im/gitterHQ/gitter.png)](https://gitter.im/mypy-django/Lobby)
+
+# django-stubs
+
+The aim of this project is to bring [PEP 484 type hints] to Django.
+
+[PEP 484 type hints]: https://www.python.org/dev/peps/pep-0484/
+
+## The Plan
+
+  * Use [retype]/[MonkeyType]/[stubgen] to auto-generate [django-stubs] Django 2.1.
+  * Integrate [machinalis/mypy-django] stubs into [django-stubs].
+  * Create a [mypy plugin] to support Django model dynamic behaviour. See [django-stubs/#5] for more.
+  * Create a test-suite to keep synced with upstream Django. See [django-stubs/#3] for more.
+  * Commence refining auto-generated stubs and plugin until it is useful.
+  * Start conversation with Django core about potential integration.
+
+This plan is not set in stone and can change.
+
+[django-stubs]: https://github.com/TypedDjango/django-stubs
+[retype]: https://github.com/ambv/retype
+[MonkeyType]: https://github.com/Instagram/MonkeyType
+[stubgen]: https://github.com/python/mypy/blob/master/mypy/stubgen.py
+[machinalis/mypy-django]: https://github.com/machinalis/mypy-django
+[django-stubs/#3]: https://github.com/TypedDjango/django-stubs/issues/3
+[mypy plugin]: https://github.com/python/mypy/tree/master/mypy/plugins
+[django-stubs/#5]: https://github.com/TypedDjango/django-stubs/issues/5
+
+## Join Us
+
+Bringing type hints to Django will be a huge achievement but will require *a
+lot* of work. It is still unclear as to how much time and effort but one thing
+is clear: we will need a lot of help. No matter what skill level or amount of
+free time, if you have some motivation to help, please do so.
+
+Right now, the best way to start helping is:
+
+  * Introduce yourself on the [Gitter chat room].
+  * Start picking up one of [the issues]
+  * Review and help triage the [project management board].
+
+[Gitter chat room]: https://gitter.im/mypy-django/Lobby
+[the issues]: https://github.com/TypedDjango/django-stubs/issues
+[project management board]: https://github.com/orgs/TypedDjango/projects/1
+
+## How was django-stubs initially auto-generated?
+
+The initial work was done by [@mkurnikov] and will be documented once [django-stubs/#2] is done.
+
+[@mkurnikov]: https://github.com/mkurnikov
+[django-stubs/#2]: https://github.com/TypedDjango/django-stubs/issues/2