Adds integrations doc. (#3929)

* Adds integrations doc.

Google, Microsoft and Dropbox for now.

* Updates doc.

doc/integrations.md

@@ -0,0 +1,38 @@
+Document describing enabling various jitsi-meet integrations.
+
+## Creating the Google API client for Google Calendar and Youtube integration
+1. Log into a Google admin account.
+1. Go to Google cloud platform dashboard. https://console.cloud.google.com/apis/dashboard
+1. In the Select a Project dropdown, click New Project.
+1. Give the project a name.
+1. Proceed to the Credentials settings of the new project.
+1. In the Credentials tab of the Credentials settings, click Create Credentials and select the type OAuth client ID.
+1. Proceed with creating a Web application and add the domains (origins) on which the application will be hosted. Local development environments (http://localhost:8000 for example) can be added here.
+1. While still in the Google cloud platform dashboard, click the Library settings for the calendar project.
+1. Search for the Google Calendar API (used for calendar accessing), click its result, and enable it.
+1. Do the same for YouTube Data API v3
+
+## Creating the Microsoft app for Microsoft Outlook integration
+1. Go to https://apps.dev.microsoft.com/
+1. Proceed through the "Add an app" flow. Once created, a page with several Graph Permissions fields should display.
+1. Under "Platforms" add "Web"
+1. Add a redirect URL for the Microsoft auth flow to visit once a user has confirmed authentication. Target domain if available is just 'yourdomain.com' (the deployment address) and the redirect URL is `https://yourdomain.com/static/msredirect.html`.
+1. Add Microsoft Graph delegated permissions, if this option is available: Calendars.Read, Calendars.ReadWrite, Calendars.Read.Shared, Calendars.ReadWrite.Shared.
+1. Check `Allow Implicit Flow` (and `Restrict token issuing to this app` if available).
+1. Save the changes.
+
+## Creating the Dropbox app for Dropbox recording integration
+1. You need a Dropbox account (If you don't already have one, you can sign up for a free account [here](https://www.dropbox.com/register).)
+1. Create new App as described in [Getting Started Guide](https://www.dropbox.com/developers/reference/getting-started?_tk=guides_lp&_ad=guides2&_camp=get_started#app%20console) in App Console section.
+1. Choose
+    1. 'Dropbox API - For apps that need to access files in Dropbox.' 
+    1. 'App folder– Access to a single folder created specifically for your app.'
+    1. Fill in the name of your app
+1. You need only, the newly created App key, goes in config.js in 
+    ```
+        dropbox: {
+            appKey: '__dropbox_app_key__'
+        }
+    ```
+1. Add your Redirect URIs in the form `https://yourdeployment.com//static/oauth.html`
+1. Fill in Branding