Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+The repository itself is simple to set up. Simply clone the repository and build it with [cargo](https://doc.rust-lang.org/cargo/getting-started/installation.html). 
+There are no extra dependencies one needs for building the repository as-is. 
+
+```sh
+git clone https://github.com/pc2/sus-compiler.git
+```
+
+And build it:
+```sh
+cargo build
+```
+
+To edit the tree-sitter grammar and regenerate [tree-sitter-sus/src/parser.c](./tree-sitter_sus/src/parser.c), you will need [`tree-sitter-cli`](https://docs.rs/tree-sitter-cli/latest/tree_sitter_cli/). 
+
+To avoid headaches due to mismatching versions, we use `tree-sitter-cli v0.24.7`
+```sh
+cargo install [email protected]
+```
+
+You can use the [tree-sitter-sus/tree.sh](tree-sitter-sus/tree.sh) script to regenerate the parser, and show some useful information about it. 
+
+## Formatting and clippy
+We require all contributions to be run through `cargo fmt` and that no warnings remain. 
+
+### VSCode
+Add the following to your `.vscode/settings.json`
+```json
+  "[rust]": {
+    "editor.defaultFormatter": "rust-lang.rust-analyzer",
+    "editor.formatOnSave": true
+  },
+  "rust-analyzer.check.command": "clippy"
+```
+
+## Behavior-altering changes
+To better defend against regressions during development, we require that any change that alters the _behavior_ of the compiler (IE, adding or changing diagnostics, or altering the test file [test.sus](test.sus)) also commits the changed effects. 
+This can be done by simply re-running 
+```sh
+./test.sus_regression.sh
+```
+Afterwards, inspect the changes to [test.sus_output.txt](./test.sus_output.txt) and [test.sus_errors.txt](./test.sus_errors.txt) before committing.