Add theme guide to contributing

Author: katieamazing

website/learn-programming-challenge/contributing.md

@@ -16,6 +16,7 @@ Halite is an open source project. We encourage you to check out our source code
 * [OPEN ISSUES ON GITHUB](https://github.com/HaliteChallenge/Halite-III/issues)
 * [COMMUNITY TOOLS](/learn-programming-challenge/downloads#community-tools)
 * [NEW STARTER KIT GUIDE](#create-a-new-starter-kit)
+* [NEW THEME GUIDE](https://forums.halite.io/t/custom-themes-for-halite-iii/737)
 
 
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