IPL Flow doc added

Author: geissonator

README.md

@@ -5,3 +5,7 @@ that's being released to support the POWER ecosystem.
 
 As always is the case with documentation, it usually needs some updates.  So feel free to do it yourself :) or touch
 base with us on the mailing lists.
+
+It is recommended that you first read the [Hostboot Programmers
+Guide](https://github.com/open-power/docs/blob/master/hostboot/HostBoot_PG.md).  This document will give you an
+overview of the Hostboot firmware and some pointers on the source code.