Create CONTRIBUTING.md

CONTRIBUTING.md

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+# How to contribute
+
+The easiest way to contribute is to open an issue and start a discussion.
+Then we can decide if and how a feature or a change could be implemented and if you should submit a pull requests with code changes.
+
+## General feedback and discussions
+Please start a discussion on the [main issue tracker](https://github.com/FluentValidation/FluentValidation/issues).
+
+## Building
+
+Run `Build.cmd` (windows) or build.sh (Linux/mac) from the command line. This builds the project and and runs tests. Building requires the following software to be installed:
+
+* Windows Powershell or Powershell Core
+* .NET Core 3.1 SDK
+* .NET Core 2.2 SDK
+* .NET Framework 4.6.1+ (Windows)
+
+## Bugs and feature requests?
+Please log a new issue in the appropriate GitHub repository:
+
+* [FluentValidation or FluentValidation.AspNetCore](https://github.com/FluentValidation/FluentValidation)
+* [Legacy ASP.NET MVC5 / WebApi 2 extensions](https://github.com/FluentValidation/FluentValidation.LegacyWeb)
+
+## Filing issues
+The best way to get your bug fixed is to be as detailed as you can be about the problem. 
+
+Please first check the documentation at https://fluentvalidation.net first to see if your question is addressed there. 
+If not, then please provide the exact version of FluentValidation that you're using along with a detailed explanation of the issue and complete steps to reproduce the problem is ideal.
+
+Please ensure all sample code is properly formatted and readable (GitHub supports [markdown](https://github.github.com/github-flavored-markdown/))
+
+## Contributing code and content
+You will need to sign a [Contributor License Agreement](https://cla.dotnetfoundation.org/) before submitting your pull request.
+
+Make sure you can build the code. Familiarize yourself with the project workflow and our coding conventions. If you don't know what a pull request is read this article: https://help.github.com/articles/using-pull-requests.
+
+If you wish to submit a new feature, please open an issue to discuss it with the project maintainers - don't open a pull request without first discussing whether the feature fits with the project roadmap.
+
+Tests must be provided for all pull requests that add or change functionality.