Contributing: Add instructions about testing the extension

CONTRIBUTING.md

@@ -11,14 +11,42 @@ You're welcome to contribute to the [code](#contributing-to-the-code) or to the
 
 ## Table of contents <!-- omit in toc -->
 
+- [Testing the extension](#testing-the-extension)
+	- [Transfering to a virtual machine](#transfering-to-a-virtual-machine)
+	- [Printing debug logs](#printing-debug-logs)
 - [Contributing to the code](#contributing-to-the-code)
 	- [Coding style](#coding-style)
 	- [Copyright notice](#copyright-notice)
+	- [Creating new files](#creating-new-files)
 	- [Localized strings](#localized-strings)
 - [Contributing to the translations](#contributing-to-the-translations)
 
 ---
 
+## Testing the extension
+
+### Transfering to a virtual machine
+
+The recommended way of testing the extension is to use a virtual machine, and to transfer the extension from your host system to the virtualized system.
+
+To pack the extension as a zip file instead of installing it on your system, you can use the `pack` option when configuring Meson:
+
+```bash
+meson builddir -Dpack=true
+```
+
+When running `meson install -C builddir`, it will create an archive in the `dist` directory. You can then transfer the extension to the VM and install it there using `gnome-extensions install nightthemeswitcher@romainvigier.fr.zip`.
+
+### Printing debug logs
+
+To print debug logs, set the build type to `debug` when configuring Meson:
+
+```bash
+meson builddir -Dbuildtype=debug
+```
+
+You can then read the logs with `journalctl -f /usr/bin/gnome-shell`.
+
 ## Contributing to the code
 
 You'll need [NPM](https://www.npmjs.com/) to install the development dependencies: