chore: update README with the description of the azure dt endpoint

Author: AndreaGiulianelli

README.md

@@ -9,4 +9,4 @@ You need to specify the following environment variable:
 - `AZURE_CLIENT_ID`: ID of an Azure AD application
 - `AZURE_TENANT_ID`: ID of the application's Azure AD tenant
 - `AZURE_CLIENT_SECRET`: the application's client secrets
-- `AZURE_DT_ENDPOINT`
+- `AZURE_DT_ENDPOINT`: the Azure Digital Twins instance endpoint