new fromkey subcommand, for registering urls, etc

had to redo Annex monad's flag storage

doc/bugs/using_url_backend.mdwn

@@ -7,3 +7,5 @@ For now, we have to manually make the symlink. Something like this:
 Note the escaping of slashes.
 
 A `git annex register <url>` command could do this..
+
+[[done]]

doc/git-annex.mdwn

@@ -4,7 +4,7 @@ git-annex - manage files with git, without checking their contents in
 
 # SYNOPSIS
 
-git annex subcommand [path ...]
+git annex subcommand [params ...]
 
 # DESCRIPTION
 
@@ -97,6 +97,16 @@ Many git-annex subcommands will stage changes for later `git commit` by you.
   Fixes up symlinks that have become broken to again point to annexed content.
   This is useful to run if you have been moving the symlinks around.
 
+* fromkey file
+
+  This can be used to maually set up a file to link to a specified key
+  in the key-value backend. How you determine an existing key in the backend
+  varies. For the URL backend, the key is just a URL to the content.
+
+  Example:
+
+	git annex fromkey --backend=URL --key=http://www.archive.org/somefile somefile
+
 # OPTIONS
 
 * --force
@@ -104,6 +114,15 @@ Many git-annex subcommands will stage changes for later `git commit` by you.
   Force unsafe actions, such as dropping a file's content when no other
   source of it can be verified to still exist. Use with care.
 
+* --backend=name
+
+  Specify the default key-value backend to use, adding it to the front
+  of the list normally configured by `annex.backends`.
+
+* --key=name
+
+  Specifies a key to operate on, for use with the addkey subcommand.
+
 ## CONFIGURATION
 
 Like other git commands, git-annex is configured via `.git/config`.

doc/walkthrough.mdwn

@@ -1,3 +1,7 @@
+A walkthrough of the basic features of git-annex.
+
+[[!toc]]
+
 ## creating a repository
 
 This is very straightforward. Just tell it a description of the repository.
@@ -130,3 +134,30 @@ Here you might --force it to drop `important_file` if you trust your backup.
 But `other.iso` looks to have never been copied to anywhere else, so if
 it's something you want to hold onto, you'd need to transfer it to
 some other repository before dropping it.
+
+## using other backends: manually adding a remote URL
+
+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
+	add 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, until you have it drop it.
+
+	# 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)