Add additional documentation on hosting and delivering updates.

README

@@ -62,6 +62,8 @@ Preparing an Update
  relevant packages, file_list.xml file and updater binary to a temporary
  directory and invoke the updater.
 
+ See doc/update-hosting for more details on hosting and delivering the updates.
+
 Invoking the Updater
 ====================
 

doc/update-hosting

@@ -0,0 +1,36 @@
+This project includes a tool for installing updates and specifies an XML-based
+file format for describing the contents of a release.  It does not include
+the client-side tools to detect the availability of updates, download
+updates that are found and invoke the update installer.  It also does
+not include the relevant server-side components.  You will need to write
+tools to do this.
+
+The simplest option is to create, for each platform, a file_list.xml file and .zip
+file containing the complete contents of the application on that platform.
+
+ 1. Modify the file_list.xml file generated by the create_packages.rb script 
+    to include <source> URLs specifying where to download the packages from and
+	then upload this modified file_list.xml file plus the compressed .zip packages to a server.
+
+ 2. The client application should then periodically fetch the file_list.xml for the latest release
+    from a fixed URL (eg. http://www.yourdomain.com/updates/$PLATFORM/file_list.xml)
+    and check whether it is newer than the installed version, by checking the <targetVersion> element in the file.
+
+ 3. If a newer version is available, the client should download the updater and .zip packages to a temporary directory
+    and invoke the updater to install the new version.
+
+A more sophisticated option is to store the file_list.xml file and packages for each release
+on the server.  When the client checks for an update:
+
+ 1. On the server, determine whether a newer version is available and if so,
+    determine the target version for the update.  If you want to have multiple 'channels'
+	of updates (eg. stable channel, beta channel, development channel) then you can change
+	the target version depending on which channel the user is in.
+
+ 2. Parse the file_list.xml for the client's current version and the
+    target version and generate a delta file_list.xml file listing only the
+    packages and files that have changed.
+
+ 3. Return the delta file_list.xml file to the client, which then downloads the necessary
+    packages and installs the updates.
+