Update contributing.md

docs/contributing.md

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 # Contributing #
 
+## Before You Start ##
+
+Drop into our [gitter chat room](https://gitter.im/Perspex/Perspex) and let us know what you're thinking of doing. We might be able to give you guidance or let you know if someone else is already working on the feature.
+
 ## Style ##
 
 The codebase uses [.net core](https://github.com/dotnet/corefx/blob/master/Documentation/coding-guidelines/coding-style.md) coding style.