Augment README.md in top level to indicate Makefile presence

Note that they are available but only meant as a guide to self building,
and are not used expressly to build as part of the overall openssl build

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
(Merged from #24047)

demos/README.txt

@@ -3,6 +3,10 @@ OpenSSL Demonstration Applications
 This folder contains source code that demonstrates the proper use of the OpenSSL
 library API.
 
+Note: Makefiles are provided in the demo subfolders to demonstrate how to build
+them, but are not frequently used.  Configure openssl with enable-demos to build
+them automatically through the perl based build system
+
 bio:                   Demonstration of a simple TLS client and server
 
 certs:                 Demonstration of creating certs, using OCSP