Java Island: Add the OVERVIEW.md for the spec

Summary: Add the OVERVIEW.md for the Java Island spec.

Test Plan: CIT passes.

Reviewed By: williamspatrick

Differential Revision: D54756494

fbshipit-source-id: 2cb9baef103cfe0e189affe4767bb16ecade9b1c

meta-facebook/spec/javaisland/OVERVIEW.md

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+## Overview
+
+Java Island is a chassis hosting four (4) slots based on the Yosemite v3.5
+(YV3.5) program.  Within the chassis is a baseboard containing AST2620 as BMC
+to provide system management for the entire Java Island unit. Each slot
+contains a Bridge IC (BIC) which is used to manage the individual slots. The
+motherboard is all a FRU with the CPU and LPDDR memory soldered to it. Java
+Island will use PLDM over MCTP over I3C as the communication interface between
+the BMC and BIC.
+
+### BMC Software
+
+The Java Island BMC is planned to run the Facebook OpenBMC software.
+
+The Facebook OpenBMC code is based on the fby35 (Yosemite v3.5) codebase and
+is kept in the meta-fby35/meta-javaisland meta-layer in the FB OpenBMC source
+tree.
+
+openBMC software will use the same utilities and checks as in prior platforms
+such as Halfdome and Craterlake.