Docs: Add RSS platform documentation

Change-Id: I2aa051f592f6a65114c073cdf0584cf36c9266f2
Signed-off-by: Jamie Fox <jamie.fox@arm.com>
diff --git a/docs/platform/ext/platform_introduction.rst b/docs/platform/ext/platform_introduction.rst
index ae03d62..2d2eb68 100644
--- a/docs/platform/ext/platform_introduction.rst
+++ b/docs/platform/ext/platform_introduction.rst
@@ -62,5 +62,9 @@
 
         - :doc:`Musca-B1 Secure Enclave. </platform/ext/target/arm/musca_b1/secure_enclave/readme>`
 
+    - Runtime Security Subsystem (RSS):
+
+        - :doc:`RSS. </platform/ext/target/arm/rss/readme>`
+
 The document :doc:`Platform Deprecation and Removal </platform/ext/platform_deprecation>`
 lists the deprecated platforms planned to be removed from upstream.