Add installation instructions

readme.md

@@ -1,4 +1,4 @@
-# Vacation Calendar
+# 🏝️ Vacation Calendar
 
 Vacation Calendar is a Google Calendar add-on that compiles your team's
 [out of office](https://support.google.com/calendar/answer/7638168?hl=en&co=GENIE.Platform%3DDesktop#:~:text=Show%20when%20you%E2%80%99re%20out%20of%20office)
@@ -12,6 +12,26 @@ the
 [out of office](https://support.google.com/calendar/answer/7638168?hl=en&co=GENIE.Platform%3DDesktop#:~:text=Show%20when%20you%E2%80%99re%20out%20of%20office)
 functionality.
 
+## Installation
+
+> [!NOTE] If you are a GitHub employee, please reach out to `@iansan5653` on Slack for the easy way
+> to get access to this addon.
+
+This addon is still under development and not yet publicly published. That said, you can still test
+it out at your own risk:
+
+1. Clone this repository on a system with Node installed
+   (`git clone https://github.com/iansan5653/vacation-calendar-addon.git`)
+2. [Create a new Google Apps Script project](https://script.google.com/home/projects/create)
+3. In your new project, navigate to **Project settings** and copy the **Script ID**
+4. In the cloned repository, update the `scriptId` field in `.clasp.json`
+5. Run `npm install`
+6. Run `npm run login` and log in to your Google account at the link provided
+7. Run `npm run push` to push the code to your project
+8. Back in your Google Apps Script project, click **Deploy**, then **Test deployments**
+9. Click **Install** and accept the permissions
+10. Navigate to Google Calendar. You should see the new icon in the right-hand sidebar
+
 ## Screenshots
 
 | Home                                                              | Calendar management                                                         |