Create Product Feedback page for Docs (#369)

* Create PRODUCT-FEEDBACK.md

* Update PRODUCT-FEEDBACK.md

PRODUCT-FEEDBACK.md

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+# Xamarin.Essentials Feedback
+
+We'd love to hear your thoughts on Xamarin.Essentials. Here are a few links to help get you to the right place:
+
+### Setup and Common Questions
+* Read through our full [Getting Started with Xamarin.Essetials Guide](https://docs.microsoft.com/xamarin/essentials/get-started)
+* [Frequently Asked Questions on our Wiki](https://github.com/xamarin/Essentials/wiki/FAQ-%7C-Essentials)
+
+### Propose a Feature
+* Read our [short guide](https://github.com/xamarin/Essentials/wiki/Proposing-New-Features) and then [file an issue](https://github.com/xamarin/Essentials/issues/new)
+
+### Report an Issue
+* Browse through [current open and closed issues first](https://github.com/xamarin/Essentials/issues?utf8=%E2%9C%93&q=is%3Aissue)
+* [File an issue](https://github.com/xamarin/Essentials/issues/new) if it hasn't been reported
+
+### Provide Feedback
+If you are using Xamarin.Essentials in your mobile application we would love your feedback. If you have a few minutes please [fill out our short survey here](https://aka.ms/essentials-feedback).