Add initial contributing doc

Basic set up instructions. Additional guidelines and documentation can
be added later.

CONTRIBUTING.md

@@ -0,0 +1,35 @@
+# Contributing
+
+Bug fixes and improvements may be submitted in the form of pull requests.
+
+## Development Setup
+
+You will need [Node.js](https://nodejs.org/download) version 10+ and
+[yarn](https://yarnpkg.com/en/docs/install).
+
+`airbrake-js` is a monorepo containing multiple packages.
+[Lerna](https://lerna.js.org/) and
+[yarn workspaces](https://yarnpkg.com/features/workspaces) are used to manage
+them. To get started, you'll need to install the project dependencies and run
+the build script:
+
+```sh
+yarn
+yarn build
+```
+
+## Building
+
+Run `yarn build` within a package directory to build that specific package, or
+run it at the project root to build all packages at once. `yarn build` must be
+run before testing or linting.
+
+## Testing
+
+Run `yarn test` within a package directory to run tests for that specific
+package, or run it at the project root to run tests for all packages at once.
+
+## Linting
+
+Run `yarn lint` within a package directory to lint that specific package, or run
+it at the project root to lint all packages at once.