docs(contr): add section on testing

Author: jsjoeio

docs/CONTRIBUTING.md

@@ -1,5 +1,6 @@
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+
 # Contributing
 
 - [Pull Requests](#pull-requests)
@@ -112,6 +113,37 @@ In our GitHub Actions CI, we use CentOS 7 for maximum compatibility.
 If you need your builds to support older distros, run the build commands
 inside a Docker container with all the build requirements installed.
 
+## Testing
+
+There are three kinds of tests in code-server:
+
+1. unit tests
+2. integration tests
+3. end-to-end tests
+
+### Unit Tests
+
+Our unit tests are written in TypeScript and run using [Jest](https://jestjs.io/).
+
+These live under [test/unit](../test/unit).
+
+### Integration Tests
+
+These are a work-in-progress. We build code-server a run a script called [test-standalone-release.sh`](../ci/build/test-standalone-release.sh)
+which ensures that code-server's CLI is working.
+
+### End-to-End Tests
+
+The end-to-end (e2e) are written in TypeScript and run using [Playwright](https://playwright.dev/).
+
+These live under [test/e2e](../test/e2e).
+
+Before the e2e tests run, we have a `globalSetup` that runs which makes it so you don't have to login before each test and can reuse the authentication state.
+
+Take a look at `codeServer.test.ts` to see how you use it (look at `test.use`).
+
+We also have a model where you can create helpers to use within tests. Take a look at [models/CodeServer.ts](../test/e2e/models/CodeServer.ts) to see an example.
+
 ## Structure
 
 The `code-server` script serves an HTTP API for login and starting a remote VS Code process.