Add Gitter badge.

I've just opened a Gitter chat room with the hopes, that I can learn femtolisp and contribute to the Julia parser.

> # Welcome fellow femto lispers!

> Welcome @/all you have all been invited to this chat room because I noticed your interest of FemtoLisp, either by chatting directly, forking, staring, watching the femtolisp repository or one of the repositories from FemtoEmacs! ✨ 

> I hope we can meet, learn and teach this programming language together, please let anyone know who might me interested about this chat room, below are some common resources that I have just recently found:

> * FemtoLisp: https://github.com/JeffBezanson/femtolisp
> * FemtoEmacs: https://github.com/FemtoEmacs/Femto-Emacs
> * FemtoDocs: https://github.com/FemtoEmacs/femtodocs

> I want to learn flisp primary in order to understand the Julia programming language parser, I hope I can contribute documenting, proof reading raising issues and pull requests.

> I wish happy new year to everyone from Mexico!

> Bless the `femtolisp` maker! - AKA @JeffBezanson :D

README.md

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+[![Join the chat at https://gitter.im/FemtoLisp/flisp](https://badges.gitter.im/FemtoLisp/flisp.svg)](https://gitter.im/FemtoLisp/flisp)
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 ## ...a purely symbolic gesture...
 
 This project began with an attempt to write the fastest lisp interpreter I could in under 1000 lines of C. It snowballed from there as I kept trying to see if I could add powerful features with minimal code. At the same time I assembled a library of some of my favorite C code (by myself and others) to use as a base for a standard library. This includes `ios`, a replacement for parts of C's stdio that adds more flexible features.