adjustments to ExtraItems

- currently only works on ground-level
- adjusted readme to inform that it doesn't work at  underground levels
- replaced the former example file that happened to be underground with one at groundlevel

gamedir/Data-1.13/TableData/Map/ExtraItems/A2_0_ExtraItems_Expert.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+   see the ExtraSectorItems-Readme.txt for details 
+-->
+<ExtraItems>
+   <Item>
+      <uiIndex>205</uiIndex>
+      <quantity>2</quantity>
+      <condition>100</condition>
+      <gridno>18174</gridno>
+      <visible>1</visible>
+   </Item>
+</ExtraItems>