Add basic README.md

Author: Immo Landwerth

README.md

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+# CodeFormatter
+
+CodeFormatter is a tool that uses Roslyn to automatically rewrite the source to
+follow our coding styles, which are [documented here][corefx-coding-style].
+
+[corefx-coding-style]: https://github.com/dotnet/corefx/wiki/Contributing#c-coding-style
+
+## Usage
+
+In order get the usage, simply invoke the tool with no arguments:
+
+```
+$ .\CodeFormatter.exe
+CodeFormatter <solution> [<rule types>] [/file <filename>]
+    <rule types> - Rule types to use in addition to the default ones.
+                   Use ConvertTests to convert MSTest tests to xUnit.
+    <filename> - Only apply changes to files with specified name.
+```
+
+## Contributing
+
+We follow the same contribution process that [corefx is using][corefx-contributing].
+
+[corefx-contributing]: https://github.com/dotnet/corefx/wiki/Contributing