Added slack badge

Author: harlanhaskins

README.md

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 # LLVMSwift
-[![Build Status](https://travis-ci.org/trill-lang/LLVMSwift.svg?branch=master)](https://travis-ci.org/trill-lang/LLVMSwift) [![Documentation](https://cdn.rawgit.com/trill-lang/LLVMSwift/master/docs/badge.svg)](https://trill-lang.github.io/LLVMSwift)
+[![Build Status](https://travis-ci.org/trill-lang/LLVMSwift.svg?branch=master)](https://travis-ci.org/trill-lang/LLVMSwift) [![Documentation](https://cdn.rawgit.com/trill-lang/LLVMSwift/master/docs/badge.svg)](https://trill-lang.github.io/LLVMSwift) [![Slack Invite](https://llvmswift-slack.herokuapp.com/badge.svg)](https://llvmswift-slack.herokuapp.com)
 
 LLVMSwift is a set of Swifty API wrappers for the LLVM C API.
 It makes compiler development feel great from Swift!