Add CONTRIBUTING.md with contribution guidelines.

Author: Azzinoth

CONTRIBUTING.md

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+## Pull Requests
+
+Please keep pull requests **relatively small and focused**. Smaller, self-contained PRs
+are much easier for me to test, validate, and review — which means they get merged faster.
+If you have a large change in mind, consider splitting it into several logical PRs.
+
+## Tests
+
+The `master` branch is kept free of tests to stay lightweight, all tests live on the
+[tests branch](https://github.com/Azzinoth/VisualNodeSystem/tree/tests).
+
+Before submitting, your new code should **pass the existing tests**.
+
+If a test fails but you believe the **test itself is flawed** (it happens), do not silently
+work around it. Instead, call it out in your pull request:
+
+- which test is failing.
+- why you think the test is wrong.
+- what you think it should check instead.
+
+## Use of AI
+
+There is **no "no-go" policy on AI**, you are welcome to use it.
+
+The one thing I ask: use AI as a targeted tool not in "vibe coding" style, and **review its output before you submit.** AI tools often introduce
+unneeded or unrelated changes that balloon a PR and make it hard to review. Trim those down
+so the diff contains only what the change actually needs, and you do not end up with a giant,
+hard-to-review pull request.
+
+## Code Style
+
+The conventions are **somewhat close to Unreal Engine's**:
+
+- **PascalCase** for types, functions, and variables (including locals and parameters).
+- **Braces: Allman** the opening brace goes on its own line.
+- **Booleans: `b` prefix everywhere** e.g. `bHovered`, `bNewValue`, `bIsValid`.
+- **Macros, compile-time constants, and enums: `ALL_CAPS_WITH_UNDERSCORES`**.
+- **Single-statement `if`/`for` bodies omit braces** put the statement on the next indented line.
+- **Tabs** for indentation.
+
+## C++ Standard
+
+The project currently targets **C++17**. You may use any language or standard-library feature up to and including C++17.