Indexes doc (#124)

README.md

@@ -815,6 +815,17 @@ This function may only be called by admins. On emulators, this function can easi
 
 None.
 
+# Indexes
+
+To perform certain queries, ENGAGE-HF requires indexes on different properties. The following indexes need to be created:
+
+|Type|Reference|Properties|Usage|
+|-|-|-|-|
+|Composite|collection:users|organization:asc,type:asc,__name__:asc|Querying users on the Web dashboard|
+|Composite|collection:invitations|user.organization:asc,user.type:asc,__name__:asc|Querying invitations on the Web dashboard|
+|Single|group:appointments|start:asc|Querying appointments across all users (for appointment messages)|
+|Single|group:devices|notificationToken:asc|Querying devices across all users (for deleting existing notification tokens assigned to other users)|
+
 # Usage
 
 To use Firebase functions for your own project or to emulate them for client applications, this section will help to give an overview of the different packages in use and how to install, build, test and launch them.