meta: Write new contribution guidelines

Author: HazAT

CONTRIBUTING.md

@@ -0,0 +1,60 @@
+<p align="center">
+  <a href="https://sentry.io" target="_blank" align="center">
+    <img src="https://sentry-brand.storage.googleapis.com/sentry-logo-black.png" width="280">
+  </a>
+  <br />
+</p>
+
+# Contributing
+
+This part of the documentation gives you a basic overview of how to help with the development of our `@sentry/*`
+packages contained in this repo.
+
+## Setting up an Environment
+
+To run the test suite and run our code linter, node.js and yarn are required.
+
+[`node`](https://nodejs.org/download) [`yarn`](https://yarnpkg.com/en/docs/install)
+
+Since this is a mono repo containing several packages, we use `lerna` to manage them.
+
+To get started call:
+
+```
+$ yarn
+```
+
+After that you need to setup the workspace with:
+
+```
+$ yarn lerna bootstrap
+```
+
+With that, the repo is fully setup and you are ready to run all commands.
+
+## Build
+
+Since we are using [`TypeScript`](https://www.typescriptlang.org/) you need to transpile the code to JavaScript to be
+able to use it. Every package has a `build` script which takes care of everything. You can also run `build` on every
+package by calling
+
+```
+$ yarn build
+```
+
+in the project root.
+
+## Running the Test Suite
+
+You can run all test at once by calling `yarn test` in the project root or in individual sub packages.
+
+## Lint
+
+You can run all test at once by calling `yarn lint` in the project root or in individual sub packages.
+
+## Contributing Back Code
+
+Please, send over suggestions and bug fixes in the form of pull requests on
+[`TypeScript`](https://github.com/getsentry/sentry-javascript). Any nontrivial fixes/features should include tests. Do
+not bump version numbers yourself. For new features and improvements consider contributing to our new SDKs instead,
+`raven-js` and `raven-node` will still be supported but are in maintenance mode and will only receive bug fixes.

README.md

@@ -15,6 +15,7 @@ backwards compatibility from time to time.
 
 ## Contents
 
+- [Contributing](https://github.com/getsentry/sentry-javascript/blob/master/README.md)
 - [Supported Platforms](#supported-platforms)
 - [Installation and Usage](#installation-and-usage)
 - [Other Packages](#other-packages)