Docs: Add contributing guide with wiki link

Author: LaCuneta

CONTRIBUTING.md

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+# Contributing to Tortoise / NetLogo Web
+
+For full contributing guidelines — including git workflow, testing, publishing Tortoise changes to Galapagos, and all code style guides (Scala, CoffeeScript, CSS) — see the **[NetLogo Web Contributing Guide](https://github.com/NetLogo/Tortoise/wiki/CONTRIBUTING)** in the Tortoise wiki.
+
+The Tortoise wiki also has detailed documentation on:
+
+- **[Building and Running](https://github.com/NetLogo/Tortoise/wiki/Building-and-Running)** — developer setup with SDKMAN! and GraalVM
+- **[Architecture](https://github.com/NetLogo/Tortoise/wiki/Architecture)** — how Tortoise, NetLogo, and Galapagos relate
+- **[Tortoise Project Setup](https://github.com/NetLogo/Tortoise/wiki/Tortoise-Project-Setup)** — sbt sub-project layout
+- **[Tortoise Tests](https://github.com/NetLogo/Tortoise/wiki/Tortoise-Tests)** — how to run tests and model dump checks
+- **[Experimental: Extensions](https://github.com/NetLogo/Tortoise/wiki/Experimental:-Extensions)** — how extensions work and how to add new ones