Update README.md

CosmosDBTrigger/README.md

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 ## Cosmos DB Trigger
-A function that will be run whenever documents change in a document collection.
+A function that will be run whenever documents change in a document collection.
+
+## How it works
+This template shows a Cosmos DB trigger binding in the function.json file and a Python function that uses that binding. The function writes log messages when Cosmos DB records are modified.