Checker Framework integration (#126)

CONTRIBUTING.md

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+Thanks for taking the time to submit a contribution to Velocity! Your support
+is greatly appreciated.
+
+In this document, we'll give you some tips on making it more likely your
+contribution will be pulled.
+
+# Setting up a development environment
+
+This isn't as difficult as you may be led to believe. All you need to do is
+clone the Velocity repository in your favorite IDE and have your backend test
+servers set up to run behind Velocity.
+
+# Actually working on the code
+
+It is strongly recommended that you are familiar with the Minecraft protocol,
+proficient with using Java, and have familiarity with the libraries used in
+Velocity (particularly [Netty](https://netty.io), [Google Guava](https://github.com/google/guava),
+and the [Checker Framework annotations](https://checkerframework.org/)).
+While you can certainly work with the Velocity codebase without knowing any
+of this, it can be risky to proceed.
+
+Velocity does not currently obey any one general code style at the moment.
+Plans are [in the works](https://github.com/VelocityPowered/Velocity/issues/125)
+to define the code style the project will follow.
+
+# Notes on the build
+
+To reduce bugs and ensure code quality, we run the following tools on all commits
+and pull requests:
+
+* [Checker Framework](https://checkerframework.org/): an enhancement to Java's type
+  system that is designed to help catch bugs. Velocity runs the _Nullness Checker_
+  and the _Optional Checker_.
+* [Checkstyle](http://checkstyle.sourceforge.net/) (not currently in use): ensures
+  that your code is correctly formatted.