docs(CONTRIBUTING): add scrip tests section

docs/CONTRIBUTING.md

@@ -1,7 +1,9 @@
 <!-- START doctoc generated TOC please keep comment here to allow auto update -->
 <!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+
 # Contributing
 
+- [Contributing](#contributing)
   - [Requirements](#requirements)
   - [Creating pull requests](#creating-pull-requests)
     - [Commits and commit history](#commits-and-commit-history)
@@ -10,6 +12,7 @@
     - [Build](#build)
   - [Test](#test)
     - [Unit tests](#unit-tests)
+    - [Script tests](#script-tests)
     - [Integration tests](#integration-tests)
     - [End-to-end tests](#end-to-end-tests)
   - [Structure](#structure)
@@ -129,13 +132,14 @@ yarn package
 > If you need your builds to support older distros, run the build commands
 > inside a Docker container with all the build requirements installed.
 
-### Test
+## Test
 
-There are three kinds of tests in code-server:
+There are four kinds of tests in code-server:
 
 1. Unit tests
-2. Integration tests
-3. End-to-end tests
+2. Script tests
+3. Integration tests
+4. End-to-end tests
 
 ### Unit tests
 
@@ -146,6 +150,14 @@ These live under [test/unit](../test/unit).
 
 We use unit tests for functions and things that can be tested in isolation. The file structure is modeled closely after `/src` so it's easy for people to know where test files should live.
 
+### Script tests
+
+Our script tests are written in bash and run using [bats](https://github.com/bats-core/bats-core).
+
+These tests live under `test/scripts`.
+
+We use these to test anything related to our scripts (most of which live under `ci`).
+
 ### Integration tests
 
 These are a work in progress. We build code-server and run a script called