Add README

Change-Id: I905240141f2f12ec65c47f1ec2b11ba20be168c3

README.md

+# MediaWiki CLI
+
+This project contains a command-line interface for interacting with MediaWiki development environments.
+
+## Docker
+
+In this initial version there is support for interacting with MediaWiki core's docker-compose development environment, 
+with subcommands provided under the `docker` namespace: `mw help docker`.
+
+## Support
+
+* Phabricator: [#MediaWiki-Docker](https://phabricator.wikimedia.org/project/view/4585/)
+* IRC: `#mediawiki`