Add Gitter badge (#4075)

diff --git a/readme.md b/readme.md
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+++ b/readme.md
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 ![logo](http://google.github.io/flatbuffers/fpl_logo_small.png) FlatBuffers
 ===========
+
+[![Join the chat at https://gitter.im/google/flatbuffers](https://badges.gitter.im/google/flatbuffers.svg)](https://gitter.im/google/flatbuffers?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Build Status](https://travis-ci.org/google/flatbuffers.svg?branch=master)](https://travis-ci.org/google/flatbuffers) [![Build status](https://ci.appveyor.com/api/projects/status/yg5idd2fnusv1n10?svg=true)](https://ci.appveyor.com/project/gwvo/flatbuffers)
 
 **FlatBuffers** is an efficient cross platform serialization library for games and