split the walkthrough and inline back together

doc/walkthrough/using_the_URL_backend.mdwn

@@ -0,0 +1,24 @@
+git-annex has multiple key-value [[backends]]. So far this walkthrough has
+demonstrated the default, WORM (Write Once, Read Many) backend. 
+
+Another handy backend is the URL backend, which can fetch file's content
+from remote URLs. Here's how to set up some files in your repository
+that use this backend:
+
+	# git annex fromkey --backend=URL --key=http://www.archive.org/somefile somefile
+	fromkey somefile ok
+	# git commit -m "added a file from the Internet Archive"
+
+Now you if you ask git-annex to get that file, it will download it, 
+and cache it locally.
+
+	# git annex get somefile
+	get somefile (downloading)
+	#########################################################################100.0%
+	ok
+
+You can always drop files downloaded by the URL backend. It is assumed
+that the URL is stable; no local backup is kept.
+
+	# git annex drop somefile
+	drop somefile (ok)