README: document output files

A grumpy BMC developer was wondering which file to flash to the machines
they were testing on. They longed for the old days where the answer was
'skiboot.lid'. Alas, those days are long past.

This updates the readme with a list of the current output formats and
where you might want to use them.

Signed-off-by: Joel Stanley <joel@jms.id.au>
[stewart: fix P8/P9 thing]
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>

README.md

@@ -123,6 +123,25 @@ You probably want to read the linux
 https://kernel.org/doc/html/latest/process/submitting-patches.html as
 much of it applies to skiboot.
 
+
+## Output files
+
+The Skiboot build process produces a bunch of different outputs. This is what
+they are, and where you should use them:
+
+ skiboot.elf: The output of the linker. Don't flash to a system, but useful when debugging
+
+ skiboot.lid: The raw binary object, named .lid because IBM. Flash this on
+	      really old P8 systems, the POWER Functional Simulator (mambo), or
+	      FSP systems
+
+ skiboot.lid.stb: Lid wrapped with secure boot header. Use on FSP systems
+
+ skiboot.lid.xz: Compressed raw binary. Use this on a OpenPower P8
+
+ skiboot.lid.xz.stb: Compressed raw binary wrapped with a secure boot header.
+                     Use this on OpenPower P9 systems
+
 ## License
 
 See LICENSE