add documentation link

cosmosdb-schema/README.md

@@ -2,7 +2,7 @@
 
 As stated in the [CosmosDB documentation][cosmosdb-limitations], it's not possible to create a unique index after a collection was created:
 
-> Azure Cosmos DB API for SQL or MongoDB accounts that create unique index after the container is created aren't supported for continuous backup. Only containers that create unique index as a part of the initial container creation are supported. For MongoDB accounts, you create unique index using extension commands.
+> Azure Cosmos DB API for SQL or MongoDB accounts that create unique index after the container is created aren't supported for continuous backup. Only containers that create unique index as a part of the initial container creation are supported. For MongoDB accounts, you create unique index using [extension commands][cosmosdb-extension-commands].
 
 The collection `qa-entries` requires a compound unique partial index to ensure the `entryIndex` numbers are numbered continuously for each talk, generating a number of QA entry numbers for each talk. This works via Mongoose schema definition of a native MongoDB instance, but not for CosmosDB with continuous backup.
 
@@ -12,3 +12,4 @@ The solution is to drop the collection `qa-entries` after creating the collectio
 
 
 [cosmosdb-limitations]: https://learn.microsoft.com/en-us/azure/cosmos-db/continuous-backup-restore-introduction#current-limitations
+[cosmosdb-extension-commands]: https://learn.microsoft.com/en-us/azure/cosmos-db/mongodb/custom-commands