Add Witherspoon Layer README

Change-Id: Ia2f1eda41b7b7af97deeb24743623fd7beecfadf
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/meta-witherspoon/README.md b/meta-witherspoon/README.md
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+Witherspoon
+================
+
+This is the Witherspoon machine layer.
+
+Witherspoon, or AC922, is an IBM POWER9 two-socket, 2U Accelerated Compute
+Server with up to 6 NVIDIA Tesla GPUs. More information can be found
+[here](https://www.ibm.com/us-en/marketplace/power-systems-ac922).