Add testing docs (#439)

* Add testing docs

* Update

.husky/pre-commit

@@ -1,4 +1,4 @@
 #!/bin/sh
 . "$(dirname "$0")/_/husky.sh"
 
-lint-staged
+npx lint-staged

README.md

@@ -82,6 +82,7 @@ function Editor() {
 ## Documentation
 
 - [How Outline was designed](/docs/design.md)
+- [Testing](/docs/testing.md)
 
 ## Contributing
 

docs/testing.md

@@ -0,0 +1,41 @@
+# Testing
+
+Outline relies on tests to ensure that changes don't break anything, using a mix of unit and end-to-end tests.
+
+## Unit tests
+
+We use Jest to run unit tests in core (`outline` package). The goal is to have a well tested API, enabling us to add or modify features without breaking it.
+
+To run the tests use:
+
+```
+npm run test-unit
+```
+
+Unit tests can be found in [this directory](/packages/outline/src/__tests__).
+
+## End-to-end (E2E) tests
+
+We use [Playwright](https://playwright.dev/) for running E2E tests in Chromium, Firefox and WebKit.
+
+These tests run in the `outline-playground` package and are divided into proactive and reactive tests (`e2e` and `regression` directories).
+
+The goal for this type of test is to validate the behavior of Outline in a browser, without necessarily knowing how the internals work.
+
+To run E2E tests use:
+
+```
+npm run start &
+npm run test-e2e:chromium # or :firefox, :webkit
+```
+
+E2E tests can be found in [this directory](/packages/outline-playground/__tests__)
+
+## General guidelines
+
+When writing tests, please follow these practices:
+
+- New features must include tests.
+- No test is too small or too big to be included. If it can break, add a test.
+- Do not merge pull requests with failing tests, this can block other people and releases.
+- Be mindful with your abstractions: sometimes it's convenient to create abstractions/utils to make test files smaller and less repetitive. Please make sure that they are simple and easy to follow.