Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to Avalonia
+
+## Before You Start
+
+Drop into our [gitter chat room](https://gitter.im/AvaloniaUI/Avalonia) 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.
+
+Try to keep lines of code around 100 characters in length or less, though this is not a hard limit.
+If you're a few characters over then don't worry too much.
+
+**DO NOT USE #REGIONS** full stop.
+
+## Pull requests
+
+A single pull request should be submitted for each change. If you're making more than one change,
+please submit separate pull requests for each change for easy review. Rebase your changes to make
+sense, so a history that looks like:
+
+* Add class A
+* Feature A didn't set Foo when Bar was set
+* Fix spacing
+* Add class B
+* Sort using statements
+
+Should be rebased to read:
+
+* Add class A
+* Add class B
+
+Again, this makes review much easier.
+
+Please try not to submit pull requests that don't add new features (e.g. moving stuff around)
+unless you see something that is obviously wrong or that could be written in a more terse or
+idiomatic style. It takes time to review each pull request - time that I'd prefer to spend writing
+new features!
+
+Prefer terseness to verbosity but don't try to be too clever.
+
+## Tests
+
+There are two types of tests currently in the codebase; unit tests and render tests.
+
+Unit tests should be contained in a class name that mirrors the class being tested with the suffix
+-Tests, e.g.
+
+    Avalonia.Controls.UnitTests.Presenters.TextPresenterTests
+
+Where Avalonia.Controls.UnitTests is the name of the project.
+
+Unit test methods should be named in a sentence style, separated by underscores, that describes in
+English what the test is testing, e.g.
+
+```csharp
+    void Calling_Foo_Should_Increment_Bar()
+```
+
+Render tests should describe what the produced image is:
+
+```csharp
+    void Rectangle_2px_Stroke_Filled()
+```
+
+## Code of Conduct
+
+This project has adopted the code of conduct defined by the Contributor Covenant to clarify expected behavior in our community.
+For more information see the [Contributor Covenant Code of Conduct](https://dotnetfoundation.org/code-of-conduct)