Add list of gems and repos for reference

[athityakumar]

We'd have to list down the gems for which we'd like to house the IRuby notebooks in the README, along with corresponding repository / documentation links. Also, open a new issue for each task (notebook) before KWoC.

[the-ethan-hunt]

Listing the gems can begin when contributions come in. Or shall we list them down so that contributors would use only these gems?

[athityakumar]

Let's list them now, so that contributors have an idea about where to refer to and what to add specifically.