Add AuthMgr README

This README will communicate Android version-specific requirements for
implementers of the AuthMgr protocol.

Bug: b/394137154
Test: N/A
Change-Id: I5843d84963bf78757b0b93b14756ff54f27b6667
diff --git a/security/see/authmgr/aidl/README.md b/security/see/authmgr/aidl/README.md
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+# AuthMgr
+
+The AuthMgr protocol authenticates and authorizes clients before they can
+access trusted HALs, AIDL-defined services in trusted execution environments.
+Version 1 was designed to allow applications running in a protected virtual
+machine (pVM) to access services running in a TEE in ARM TrustZone. An
+implementation of `IAuthMgrAuthorization` is referred to as an AuthMgr Backend.
+An implementation of a client of the AuthMgr Backend is referred to as an
+AuthMgr Frontend.
+
+
+## Additional Requirements by Android Version
+
+The comments on `IAuthMgrAuthorization` describe the requirements for implementing
+an AuthMgr Backend (implementor of the interface) itself. There are some additional
+requirements that are specific to Android release versions.
+
+### Android 16
+If implementing `IAuthMgrAuthorization` in Android 16 only one AuthMgr Backend is
+supported and dynamic service discovery is not supported. The AuthMgr Backend
+service must be exposed on secure partition ID 0x8001 over VSOCK port 1.
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