Add gitter badge

[koppor]

This addresses #3586

In the DevCall, we decided, that we will try to use Gitter and see how it goes.

The main arguments were:

• easy to join for new users
• offer help for new developers since the burden of initial JabRef development is high for newcomers - even if we have https://github.com/JabRef/jabref/wiki/High-Level-Documentation and https://github.com/JabRef/jabref/wiki/Code-Howtos
• Nearly none of the developers uses Slack
• does not clutter the Skype chat of the developers
• developers can use pinned tabs to keep gitter always opened

We will shutdown gitter in case there won't be much support from @JabRef/developers there (because lack of time). Nevertheless, we give it a chance. 😇

[lenhard]

Since testing gitter has been tentatively decided at the devcall, I'll merge this right away.

[tobiasdiez]

The gitter chat is meant as a platform to discuss developer's question and not to give user support, right? In this case, I would suggest to follow the rails example and create a contributing section in the readme with the two badges "code helpers" and "gitter" to make the context clear. In this new section, we could also add links to the contributing FAQ and the related wiki pages to make contributing even easier.

[lenhard]

Yes, you are right: gitter is for developer's questions. Maybe I was to quick with merging, apologies!