ADD: Testing documentation

README.md

@@ -769,6 +769,10 @@ $ grunt concat:dist
 
 3. _Optional_: We use SVG for our icon system. Please visit our wiki [SVG Icon System](https://github.com/publiclab/Leaflet.DistortableImage/wiki/SVG-Icon-System) if you are interested in making updates to them or just simply learning about our workflow.
 
+### Testing
+
+[Guide](TESTING.md) on testing LDI.
+
 ---
 
 ### Contributors

TESTING.md

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+## Testing
+
+We use Mocha & Karma with the help of Grunt for testing.
+
+To learn more about these tools, visit the official docs:
+
+- [Mocha](https://mochajs.org/#getting-started)
+- [Karma](https://karma-runner.github.io/4.0/config/configuration-file.html)
+- [Grunt](https://gruntjs.com/getting-started)
+
+`npm test` runs tests from the `test/` folder.
+
+
+
+`npm test` is just a wrapper around `grunt test` task which runs JSHint and Karma tasks.
+
+[JSHint](https://jshint.com/docs/) is a JavaScript code quality tool, an alternative to [ESLint](https://eslint.org/).
+
+[Karma](https://karma-runner.github.io/latest/index.html) is a test runner, it allows us to test our code in multiple browsers and devices.
+
+
+
+### Running just one test
+
+For this, you need to install Mocha globally. Run `npm i -g mocha`
+
+After you've successfully installed Mocha, you can run just one type of test by passing a test file to Mocha:
+
+`mocha <filename>.js`
+
+
+
+### Running a single test block within a test file
+
+In order to do this, you don't need to install Mocha globally.
+
+You can just attach a `.only()` function call to a test block that you want to execute.
+
+It can be attached to both, `describe` and `it` test blocks.
+
+
+
+**EXAMPLE**:
+
+```javascript
+it('tests feature 1', function(){
+    // I won't run :(
+});
+
+it.only('tests feature 2', function(){
+    // I'll run :)
+});
+
+it('tests feature 3', function(){
+    // I won't run :(
+});
+```
+
+
+
+### Skipping tests
+
+Sometimes, we just want to write a boilerplate code for a test, but we don't want to implement it yet.
+
+We can attach `.skip()` function call, which will skip our `it` or `describe` blocks, works just like `.only()`.
+
+
+
+**NOTE**: After you finish writing a test, remember to remove your `.only()`or `.skip()` calls.
+
+They are meant for development purposes, in the production we don't want to alter the actual code of our tests.