Add badges and a "under development" notice to README.md

Summary:
Add a notice to the README.md that this lib is still under heavy development and not usable until now.
Also added some badges to show the build status of travis and some statistics from packagist.

Reviewers: dkd-doan, #forgetit-dev, claus.due

Reviewed By: #forgetit-dev, claus.due

Differential Revision: https://phabricator.dkd.de/D383