Added a contributing guide

Happy Hacktoberfest 🎃

CONTRIBUTING.md

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+# Contributing
+
+Discord.Net is an open-source project, and we appreciate any and all
+contributions made by our community. However, please conform to the
+following guidelines when possible:
+
+## Development Cycle
+
+We prefer all changes to the library to be discussed beforehand,
+either in a GitHub issue, or in a discussion in our Discord channel
+with library regulars or other contributors.
+
+Issues that are tagged as "up for grabs" are free to be picked up by
+any member of the community.
+
+### Pull Requests
+
+We prefer pull-requests that are descriptive of the changes being made
+and highlight any potential benefits/drawbacks of the change, but these
+types of write-ups are not required. See this [merge request](https://github.com/RogueException/Discord.Net/pull/793)
+for an example of a well-written description.
+
+## Semantic Versioning
+
+This project follows [Semantic Versioning](http://semver.org/). When
+writing changes to this project, it is recommended to write changes
+that are SemVer compliant with the latest version of the library in
+development.
+
+The working release should be the latest build off of the `dev` branch,
+but can also be found on the [development board](https://github.com/RogueException/Discord.Net/projects/1).
+
+We follow the .NET Foundation's [Breaking Change Rules](https://github.com/dotnet/corefx/blob/master/Documentation/coding-guidelines/breaking-change-rules.md)
+when determining the SemVer compliance of a change.
+
+Obsoleting a method is considered a **minor** increment.
+
+## Coding Style
+
+We attempt to conform to the .NET Foundation's [Coding Style](https://github.com/dotnet/corefx/blob/master/Documentation/coding-guidelines/coding-style.md)
+where possible.
+
+As a general rule, follow the coding style already set in the file you
+are editing, or look at a similar file if you are adding a new one.