docs(test): add instruction on how to integrate test in pre-commit hook

packages/react-scripts/template/README.md

@@ -51,14 +51,15 @@ You can find the most recent version of this guide [here](https://github.com/fac
 - [Running Tests](#running-tests)
   - [Filename Conventions](#filename-conventions)
   - [Command Line Interface](#command-line-interface)
-  - [Version Control Integration](#version-control-integration)
   - [Writing Tests](#writing-tests)
   - [Testing Components](#testing-components)
   - [Using Third Party Assertion Libraries](#using-third-party-assertion-libraries)
   - [Initializing Test Environment](#initializing-test-environment)
   - [Focusing and Excluding Tests](#focusing-and-excluding-tests)
   - [Coverage Reporting](#coverage-reporting)
+  - [Version Control Integration](#version-control-integration)
   - [Continuous Integration](#continuous-integration)
+  - [Pre-commit Hook](#pre-commit-hook)
   - [Disabling jsdom](#disabling-jsdom)
   - [Snapshot Testing](#snapshot-testing)
   - [Editor Integration](#editor-integration)
@@ -1084,14 +1085,6 @@ The watcher includes an interactive command-line interface with the ability to r
 
 ![Jest watch mode](http://facebook.github.io/jest/img/blog/15-watch.gif)
 
-### Version Control Integration
-
-By default, when you run `npm test`, Jest will only run the tests related to files changed since the last commit. This is an optimization designed to make your tests run fast regardless of how many tests you have. However it assumes that you don’t often commit the code that doesn’t pass the tests.
-
-Jest will always explicitly mention that it only ran tests related to the files changed since the last commit. You can also press `a` in the watch mode to force Jest to run all tests.
-
-Jest will always run all tests on a [continuous integration](#continuous-integration) server or if the project is not inside a Git or Mercurial repository.
-
 ### Writing Tests
 
 To create tests, add `it()` (or `test()`) blocks with the name of the test and its code. You may optionally wrap them in `describe()` blocks for logical grouping but this is neither required nor recommended.
@@ -1232,6 +1225,14 @@ Run `npm test -- --coverage` (note extra `--` in the middle) to include a covera
 
 Note that tests run much slower with coverage so it is recommended to run it separately from your normal workflow.
 
+### Version Control Integration
+
+By default, when you run `npm test`, Jest will only run the tests related to files changed since the last commit. This is an optimization designed to make your tests run fast regardless of how many tests you have. However it assumes that you don’t often commit the code that doesn’t pass the tests.
+
+Jest will always explicitly mention that it only ran tests related to the files changed since the last commit. You can also press `a` in the watch mode to force Jest to run all tests.
+
+Jest will always run all tests on a [continuous integration](#continuous-integration) server or if the project is not inside a Git or Mercurial repository.
+
 ### Continuous Integration
 
 By default `npm test` runs the watcher with interactive CLI. However, you can force it to run tests once and finish the process by setting an environment variable called `CI`.
@@ -1248,6 +1249,7 @@ Popular CI servers already set the environment variable `CI` by default but you
 ```
 language: node_js
 node_js:
+  - 4
   - 6
 cache:
   directories:
@@ -1292,6 +1294,39 @@ The test command will force Jest to run tests once instead of launching the watc
 
 The build command will check for linter warnings and fail if any are found.
 
+### Pre-commit Hook
+
+You can run tests against "staged" files before each Git commit by integrating the test script in pre-commit hook.
+
+First, install [husky](https://github.com/typicode/husky) & [lint-staged](https://github.com/okonet/lint-staged):
+```sh
+npm install --save-dev husky lint-staged
+```
+
+Because we don't need the tests to run in watch mode, we need to set `CI` environment variable to `true`. As described in section [Continuous Integration](#continuous-integration).
+
+To make sure it's cross-platform, let's install [cross-env](https://github.com/kentcdodds/cross-env):
+```sh
+npm install --save-dev cross-env
+```
+
+Then add this config to `package.json`:
+```
+"scripts": {
+  ...
+  "precommit": "lint-staged",
+  "test:staged": "cross-env CI=true react-scripts test --env=jsdom --findRelatedTests"
+},
+"lint-staged": {
+  "src/**/*.js": [
+    "test:staged",
+    "git add"
+  ]
+}
+```
+
+This way, instead of running all tests, passing `--findRelatedTests` flag in test script will save our times a lot because Jest will run only the minimal amount of tests related to changes in your staging area.
+
 ### Disabling jsdom
 
 By default, the `package.json` of the generated project looks like this: