[PATCH] Document subproject feature

@@ -1,4 +1,4 @@
-Git User's Manual (for version 1.5.1 or newer)
+Git User's Manual (for version 1.5.2 or newer)
 ______________________________________________
 
 This manual is designed to be readable by someone with basic unix

@@ -1406,6 +1406,39 @@ just performs a "fast forward"; the head of the current branch is moved
 forward to point at the head of the merged-in branch, without any new
 commits being created.
 
+[[subprojects]]
+Subprojects
+-----------
+
+Some large development efforts, such as embedded Linux distributions,
+are composed of a set of large projects, each with its own development
+team, but all of which are combined to produce the project as a whole.
+For example, there might be a firmware project, a hypervisor project,
+a kernel project, and a userspace project.  Note that while each
+project is conceptually independent, there are many cases in which a
+change to the hypervisor necessitates a change to the kernel, for
+example.
+
+In this case it is nice to be able to reason about the state of the
+entire project, but also not inconvenience each development team with
+checking out a gigantic repository that represents the entire project.
+Git provides subproject support for this case, which is similar to CVS
+modules or the hg forest extension.
+
+Here is an example of creating a subproject inside an existing project:
+
+-------------------------------------------------
+$ mkdir subproject
+$ cd subproject
+$ git init
+$ touch Makefile
+$ git add Makefile
+$ git commit -m "Create subproject."
+$ cd ..
+$ git add subproject
+$ git commit -m "Add subproject."
+-------------------------------------------------
+
 [[fixing-mistakes]]
 Fixing mistakes
 ---------------