Update README.md

Remove Gitter since the link is 404 and we have not been using it anyway.

Author: tritao

README.md

@@ -76,8 +76,6 @@ Please see the following resources for more information:
 
 Feel free to open up issues on Github for any problems you find.
 
-You can also join us at our [#managed-interop](https://gitter.im/managed-interop) Gitter discussion channel.
-
 ## Support
 
 For building wrappers and priority support please write to cppsharp@protonmail.com.