Added dependency badge for README file;

Hi, 

i added dependency badge into README. This badge helps to keep eye on sub-dependencies (specially before the release 4.0) and users can checkout additional metadata about your library. For example, here is [list of libraries](https://www.versioneye.com/ruby/mongoid/references) which are  already using `Mongoid`.

Seems that tzinfo-guys are done huge leap in their versioning - directly from [`0.3` to `1.0`](https://github.com/tzinfo/tzinfo/blob/master/CHANGES.md)  .