Documentation: Add FIT payload documentation

Describe the new uImage.FIT loader.

Change-Id: I8b2060f2a63406669196bcbc190cc1511ae9fe94
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/27253
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>

Documentation/lib/payloads/index.md

@@ -0,0 +1,11 @@
+# Payload-specific documentation
+
+This section contains documentation about coreboot on specific payloads.
+
+Payloads are run after coreboot has initialized the hardware.
+coreboot supports multiple payloads, depending on the architecture of the
+selected mainboard.
+
+## FIT
+
+- [uImage.FIT support](fit.md)