Add contributing guide

CONTRIBUTING.md

@@ -0,0 +1,25 @@
+# Contributing Guide
+
+Hey there! So you like WorldEditAdditions enough to consider helping out? That's awesome! This guide should get you up and running in no time.
+
+
+## Code structure
+The WorldEditAdditions codebase is split into 3 main submods:
+
+Name							| Description
+--------------------------------|------------------------
+`worldeditadditions`			| The main mod. Core world manipulation implementations should go in here.
+`worldeditadditions_commands`	| Chat commands. These interact with the core manipulators in `worldeditadditions` mod.
+`worldeditadditions_farwand`	| Everything to do with the far wand tool. It's different enough to everything else that it warrants it's own separate mod to avoid muddling things.
+
+Additionally, every command should be implemented in its own file. This helps keep things organised and files short.
+
+## Guidelines
+When actually implementing stuff, here are a few guidelines that I recommend to summarise everything:
+
+ - Keep each command implementation to its own file
+ - Split up files with more than 500 lines into smaller chunks (such as what I've done with the `//convolve` implementation in the `worldeditadditions` submod)
+ - Try to follow the existing programming style
+ - If you think of something helpful to add to this guide, please open an issue / PR :D
+ - Being excellent to everyone shouldn't have to be on this list, but it is
+ - @sbrl has the final say