Clarify in README that v0.9 and v1.0 are deprecated

Change-Id: Ibff46e7d88fcec0b1f3352aa7cebd7f5a7204e47
Signed-off-by: Jose Marinho <jose.marinho@arm.com>

Author: jmarinho

README.md

@@ -6,6 +6,8 @@ This repository contains the Firmware Handoff specification, which defines a
 data structure to transfer essential configuration information between firmware
 stages during platform initialization.
 
+Note that versions 0.9 and 1.0 of this specification were deprecated and should not be used for product development.
+
 The documentation is generated using the Sphinx framework. A version of this
 specification, rendered in HTML, is available
 [here](https://firmwarehandoff.github.io/firmware_handoff/).