Add a bit of documentation on how to run the code coverage (#25783)

js/tests/README.md

@@ -27,16 +27,20 @@ To run the unit test suite via a real web browser, open `index.html` in the brow
 * Each test should begin with [`assert.expect`](https://api.qunitjs.com/assert/expect/) to ensure that the expected assertions are run.
 * Each test should follow the project's [JavaScript Code Guidelines](https://github.com/twbs/bootstrap/blob/master/CONTRIBUTING.md#js)
 
+## Code coverage
+
+Currently we're aiming for at least 80% test coverage for our code. To ensure your changes meet or exceed this limit, run `npm run js-compile && npm run js-test` and open the file in `js/coverage/lcov-report/index.html` to see the code coverage for each plugin. See more details when you select a plugin and ensure your change is fully covered by unit tests.
+
 ### Example tests
 
-```javascript
+```js
 // Synchronous test
 QUnit.test('should describe the unit being tested', function (assert) {
   assert.expect(1)
-  var templateHTML = '<div class="alert alert-danger fade show">'
-      + '<a class="close" href="#" data-dismiss="alert">×</a>'
-      + '<p><strong>Template necessary for the test.</p>'
-      + '</div>'
+  var templateHTML = '<div class="alert alert-danger fade show">' +
+        '<a class="close" href="#" data-dismiss="alert">×</a>' +
+        '<p><strong>Template necessary for the test.</p>' +
+        '</div>'
   var $alert = $(templateHTML).appendTo('#qunit-fixture').bootstrapAlert()
 
   $alert.find('.close').trigger('click')