doc(contributing): add end-to-end tests explanations

CONTRIBUTING.md

@@ -28,11 +28,12 @@ the various packages, please execute the following:
 make init
 ```
 
-> please note that it will take a while as this project uses a lot of dependencies…'
+> please note that it will take a while as this project uses a lot of dependencies…
 
 ### Windows
 
-If you want to build this project on Windows, it is recommended to use either WSL 2, or Git bash + `choco install make`.
+If you want to build this project on Windows, it is recommended to use either WSL 2,
+or Git bash + `choco install make`.
 
 ## Development
 
@@ -77,6 +78,13 @@ You can also build the packages without running a watcher, you have two options:
 
 ### Testing
 
+#### Unit tests
+
+Unit tests for each package are located in the `packages/<pkg>/tests` folder, we're using jest
+as a test running and `react-test-renderer` as a testing library, some tests are still using
+`enzyme`, but this lib is not maintained anymore and doesn't support newer versions of React,
+so those should eventually be migrated.
+
 To run unit tests on all packages, run the following command:
 
 ```
@@ -92,6 +100,14 @@ make pkg-test-bar
 
 where `bar` is the name of the targeted nivo package.
 
+#### End-to-end tests
+
+Sometimes it's difficult to test certain things in unit tests, from our experience, animations
+and interactions can be tricky to test via unit tests only, so we also try to have end-to-end tests.
+
+We're using `cypress` for writing such tests, and those are located in the `cypress/src/components`
+folder, as we introduced end-to-end tests later, not all packages have some.
+
 ### Formatting
 
 Nivo uses prettier in order to provide a consistent code style.