Contributing guidelines now say how to compile it

As per https://github.com/NyxStudios/TerrariaAPI-Server/pull/41#issuecomment-150354612, we now have instructions on how to compile TShock and the API in simple enough terms that anyone familiar with .NET can understand. If not, we have a note to drop by Slack.

CONTRIBUTING.md

@@ -6,6 +6,18 @@ Please follow these simple requirements before posting a bug report:
 - How to reproduce the issue
 - Screenshots of the issue (if applicable)
 
+### To build the source
+
+Note: This includes the API by default. If you need only the API, you need to cd into that folder and do the following with the .sln file for the API. For those new to C#, the .sln and .csproj files contain the necessary definitions to do a complete source build using Microsoft or Mono build tools.
+
+- Checkout the source.
+- Initialize the submodules: ```git submodule init```
+- Open the source in your favorite text editor that supports .NET building and press the build button OR
+- Run ```msbuild TShock.sln``` in the root of the cloned folder on Windows in a 'Developer Command Prompt' OR
+- Run ```xbuild TShock.sln``` in the root of the cloned folder on Unix.
+
+Need help? Drop by Slack and we'll be happy to explain it with more words, step by step.
+
 ### TShock Additions
 
 If something is better suited to be a plugin for TShock, rather than a TShock core feature, it should not be added!