Create CONTRIBUTING.md

.github/CONTRIBUTING.md

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+# Guidelines for contributions
+
+## Development Requirements
+
+Recommended IDE is vscode.
+
+### Linux
+
+`yay -Syu shaderc spirv-cross vulkan-devel`
+
+(vulkan-devel is optional)
+
+### Windows
+
+* shaderc (glslc)
+* spirv-cross
+* (optional) vulkan sdk
+
+## Pull Requests
+
+* Don't make changes to the main branch in your fork.
+* Split unrelated changes into separate PRs
+* Branch name format for syncing to the new Ultralight version is `upstream_yyyy_mm_dd_hash`