adding Contributing guide

CONTRIBUTING.md

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+# Contributing to Boa
+
+Boa welcomes contribution from everyone. Here are the guidelines if you are
+thinking of helping out:
+
+## Contributions
+
+Contributions to Boa or its dependencies should be made in the form of GitHub
+pull requests. Each pull request will be reviewed by a core contributor
+(someone with permission to land patches) and either landed in the main tree or
+given feedback for changes that would be required. All contributions should
+follow this format.
+
+Should you wish to work on an issue, please claim it first by commenting on
+the GitHub issue that you want to work on it. This is to prevent duplicated
+efforts from contributors on the same issue.
+
+Head over to [issues](https://github.com/jasonwilliams/boa/issues) and check for "good first issue" labels to find
+good tasks to start with. If you come across words or jargon that do not make
+sense, please ask!
+
+If you don't already have Rust installed rustup is the recommended tool to use. It will install Rust and allow you to switch between nightly, stable and beta. You can also install additional components.
+
+`curl https://sh.rustup.rs -sSf | sh`
+
+Then simply clone this project and `cargo build`
+
+### Web Assembly
+
+If you want to develop on the web assembly side you can run yarn serve then go to http://localhost:8080
+
+### Setup
+
+VScode is commonly used with the [Rust (RLS) plugin](https://github.com/rust-lang/rls-vscode).