Replace gitter chat with link to slack - closes #2975 (#2978)

Author: forki

README.md

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-[![Join the chat at https://gitter.im/Microsoft/visualfsharp](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/Microsoft/visualfsharp?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
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 # The Combined F# Language, Library, and Tools Repository
 
 You are invited to to help produce future releases of the F# language compiler, library, and tools.
@@ -46,8 +43,6 @@ This repository enables development on Linux, macOS and Windows.  It enables aut
 
 Changes contributed here are eventually included in all packagings of F# and open source F# editing tools. Microsoft coordinates packaging this repository as part of the Visual F# Tools, and the F# community coordinates packaging [other editions of F#](https://github.com/fsharp/fsharp/) for use on Linux, macOS, Android, iOS, and other platforms.
 
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 ## Using
 
 For typical installs of  F#, see http://fsharp.org.
@@ -70,13 +65,12 @@ The actual package is built in https://github.com/fsharp/fsharp.
 You will need to adjust the targets reference on your project file to use the targets file from the installed ``FSharp.Compiler.Tools`` package.
 See https://github.com/fsharp/fsharp/issues/676 for how to modify your project file.
 
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 ## Code of Conduct
 
 This project has adopted the code of conduct defined by the [Contributor Covenant](http://contributor-covenant.org/) to clarify expected behavior in our community. This code of conduct has been [adopted by many other projects](http://contributor-covenant.org/adopters/). For more information see the [Code of conduct](https://github.com/Microsoft/visualfsharp/wiki/Code-of-Conduct).
 
 ## Get In Touch
 
-Follow [@VisualFSharp](https://twitter.com/VisualFSharp) and [@fsharporg](https://twitter.com/fsharporg) on twitter and  subscribe to the [.NET Blog](https://blogs.msdn.microsoft.com/dotnet/).
+Follow [@VisualFSharp](https://twitter.com/VisualFSharp) and [@fsharporg](https://twitter.com/fsharporg) on twitter and subscribe to the [.NET Blog](https://blogs.msdn.microsoft.com/dotnet/).
 
+Members of the F# Software Foundation can be invited to the "F# Software Foundation" discussion rooms on slack. More details at http://fsharp.org/guides/slack/.