Create ch10-app-secrets.md

Author: markjprice

docs/ch10-app-secrets.md

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+**App Secrets**
+
+- [What is an app secret?](#what-is-an-app-secret)
+
+# What is an app secret?
+
+Never store a password or other sensitive values in your source code. Get those values from an environment variable or a secret management system like: 
+- [Secret Manager](https://learn.microsoft.com/en-us/aspnet/core/security/app-secrets) for local development secrets.
+- [Azure Key Vault](https://learn.microsoft.com/en-us/azure/key-vault/general/overview) for cloud and production secrets.
+