[#449] [9.1] Remove explicit file list from build\assets\readme

* So it only needs to be maintained in one place

build/assets/readme.txt

@@ -1,14 +1 @@
-Add asset files to this directory. You will need:
-
-- license.xml (your Sitecore license)
-- Sitecore 9 OnPrem WDP packages
-	Sitecore 9.0.2 rev. 180604 (OnPrem)_single.scwdp.zip
-	Sitecore 9.0.2 rev. 180604 (OnPrem)_xp0xconnect.scwdp.zip
-- from the XP0 config files that come with the OnPrem WDP packages:
-	xconnect-solr.json
-	xconnect-createcert.json
-	xconnect-xp0.json
-	sitecore-solr.json
-	sitecore-xp0.json
-
-See the documentation for more details.
+Add install asset files to this directory. See the documentation for details.