PEL: Document UserData sections

Add a section in the PEL README to document the UserData sections
that will be created in every new PEL.

Change-Id: I9ff9a69f1d22b56aa8bb9d559802d0595d787c0a
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/extensions/openpower-pels/README.md b/extensions/openpower-pels/README.md
index bb47cce..14fade2 100644
--- a/extensions/openpower-pels/README.md
+++ b/extensions/openpower-pels/README.md
@@ -41,6 +41,15 @@
 _PID="12345"
 ```
 
+## Default UserData sections for BMC created PELs
+
+The extension code that creates PELs will add these UserData sections to every
+PEL:
+
+- The AdditionalData property contents
+  - If the AdditionalData property in the OpenBMC event log has anything in it,
+    it will be saved in a UserData section as a JSON string.
+
 ## The PEL Message Registry
 
 The PEL message registry is used to create PELs from OpenBMC event logs.