docs: add TF-A version numbering information

Add a chapter "Version numbering" in release information file that
explains macros used for TF-A version. It also introduces VERSION_PATCH
macro that is used for LTS releases. A comment for this macro is also
added in Makefile.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I946b6cb91bb8454131f07b24534d28ab1aef1771
diff --git a/Makefile b/Makefile
index 640060b..f55dfdd 100644
--- a/Makefile
+++ b/Makefile
@@ -9,7 +9,7 @@
 #
 VERSION_MAJOR			:= 2
 VERSION_MINOR			:= 9
-VERSION_PATCH			:= 0
+VERSION_PATCH			:= 0	# Only used for LTS releases
 VERSION				:= ${VERSION_MAJOR}.${VERSION_MINOR}.${VERSION_PATCH}
 
 # Default goal is build all images