documentation: define downstream data consumption rules

It's important to document what data is available for
downstream consumption by the OS and payloads. Provide
details/directions on what is available.

Change-Id: Icafb6e9f59462ad19cf6a641046c6efc36f138f0
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/9194
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marc.jones@se-eng.com>

documentation/abi-data-consumption.txt

@@ -0,0 +1,22 @@
+This text describes the ABI coreboot presents to downstream users. Such
+users are payloads and/or operating systems. Therefore, this text serves
+at what can be relied on for downstream consumption. Anything not explicitly
+listed as consumable is subject to change without notice.
+
+Background and Usage
+
+coreboot passes information to downstream users using coreboot tables. These
+table definitions can be found in src/include/boot/coreboot_tables.h and
+payloads/libpayload/include/coreboot_tables.h respectively within coreboot
+and libpayload. One of the most vital and important pieces of information
+found within these tables is the memory map of the system indicating
+available and reserved memory.
+
+In 2009 cbmem was added to coreboot. The "CBMEM high table memory manager"
+serves a way for coreboot to bookkeep its own internal data. While some
+of this data may be exposed through the coreboot tables the data structures
+used to manage the data within the cbmem area is subject to change.
+
+Provided the above, if one needs a piece of data exposed to the OS
+or payload it should reside within the coreboot tables. If it isn't there
+then a code change will be required to add it to the coreboot tables.