Replace mySociety README with a GOV.UK specific one

For GOV.UK apps the README is usually a quick reference for developers who
may not be familiar with the project.

Add a new README that follows the GOV.UK README styleguide [1], aiming to
orient those new to MapIt and its use in GOV.UK.

The original (`.rst`) README was removed as GH still picks that over the
new (`.md`) if both exist. To avoid confusion the new links to the old
on the mySociety repo.

[1]: https://github.com/alphagov/styleguides/

README.md

@@ -0,0 +1,39 @@
+# MapIt
+
+A web service to map postcodes to administrative boundaries and more.
+
+MapIt was created and is maintained by [mySociety](https://github.com/mysociety/mapit), it's recommended you read their [README](https://github.com/mysociety/mapit/blob/master/README.rst) as well.
+
+## Development
+
+This fork of MapIt should avoid adding features, changing behaviours, or touching the code of MapIt itself. The only [differences against the original repo](https://github.com/mysociety/mapit/compare/master...alphagov:master) should only be to help integrate it into the GOV.UK stack. This allows our fork to be kept up to date with the mySociety version.
+
+So far this includes;
+
+ - [Pinning to specific versions of Python dependencies](https://github.com/alphagov/mapit/pull/1), for reliability
+ - [Adding a Procfile](https://github.com/alphagov/mapit/pull/2), to standardise deployment with other apps
+ - [A GOV.UK specific README](https://github.com/alphagov/mapit/pull/3), to provide context for GOV.UK developers
+
+## Technical documentation
+
+MapIt is a Python/Django application (backed by PostgreSQL), that provides
+RESTful API for looking up postcodes, council boundaries, etc.
+
+### Dependencies
+
+MapIt has no dependencies on the rest of the GOV.UK stack, but does use PostgreSQL
+with some geo-spatial extensions. For the GOV.UK VM these are documented in [puppet](https://github.gds/gds/puppet/blob/master/modules/govuk/manifests/apps/mapit.pp), or you can follow the [standalone install guide](http://mapit.poplus.org/docs/self-hosted/install/).
+
+### Running the application
+
+You'll need to be in a [virtualenv](http://docs.python-guide.org/en/latest/dev/virtualenvs/), and install dependencies with:
+
+```pip install -r requirements.txt```
+
+You can then run an instance of mapit with:
+
+```manage.py runserver 127.0.0.1:3108```
+
+## Licence
+
+[GNU Affero GPL](LICENCE.txt)