Add CONTRIBUTING.md

Author: polybuildr

CONTRIBUTING.md

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+# Contributors Guide
+
+First off, thanks a lot for contributing to Balloon! :D
+
+## Setup
+
+Balloon is written in Rust. You'll need to get a working installation of Rust and Cargo to be able to work on Balloon. The [Rust websites's installation instructions page](https://www.rust-lang.org/en-US/install.html) should give you the necessary information. Remember to configure the `$PATH` environment variable (if necessary) as mentioned in the guide. You can get the latest nightly version of the compiler or the stable version - balloon should build fine in both.
+
+After this, you can run the REPL:
+
+```
+$ cargo run
+```
+
+You can also build the binary and use it directly as mentioned in the [readme](README.md).
+
+```
+$ cargo build
+$ ./target/debug/balloon [args]
+```
+
+If you want to work on the LLVM backend, you'll need to install LLVM (look at the apt packages listed in [.travis.yml](.travis.yml)) and then run cargo with the "llvm-backend" feature.
+
+```
+$ cargo build --features "llvm-backend"
+```
+
+## Project structure for contributions
+
+Say you want to add a new language construct to Balloon. The process looks roughly as follows:
+
+1. Make the necessary changes to the `struct`s and `enum`s in the [src/ast.rs](src/ast.rs) file.
+1. Edit the PEG grammar file at [src/grammar.rustpeg](src/grammar.rustpeg).
+1. Try building once using `cargo build`. If there's an error in the grammar or the AST, it'll show up now. If those parts are fine, you'll see an error because some case isn't handled in [src/ast_walk_interpreter.rs](src/ast_walk_interpreter.rs) and in [src/typechecker.rs](src/typechecker.rs).
+1. Find the relevant cases in the AST-walk interpreter and the typechecker and add in the relevant code.
+1. Build and test the feature manually to see if it works. (Unless you're doing something cool like TDD.)
+1. Write tests. Look at the [tests/](tests/) directory for examples.
+    - run-pass: Files that should run without errors
+    - run-fail: Files that should given an error when run, specified in the respective .err file
+    - typecheck-pass: Files that should typecheck without errors
+    - typecheck-fail: Files that should give an error when typechecked, specified in the respective .err file
+1. If everything looks good, commit and push your change. While there aren't any concrete rules yet, follow the commonly used imperative style, without a trailing period. Look at previous commits to get an idea of how your commit should look.
+1. Send in a PR!