INSTALL.md: document how to build from git (#20)

Fixes https://github.com/aspiers/stow/issues/20

INSTALL.md

@@ -32,6 +32,26 @@ Advantages of the `Module::Build` approach:
   - It performs dependency checking to ensure you have the necessary
     Perl modules installed.
 
+Both approaches are described in detail below.  However if you are
+building from the git repository rather than an official release,
+you first need to perform some extra steps:
+
+Preparatory steps required only when building from git
+------------------------------------------------------
+
+`configure` and `Makefile` are included in official releases of Stow,
+but they are deliberately omitted from the git repository because they
+are autogenerated.  Therefore if you are installing directly from git,
+you first need to generate them as follows.
+
+First `cd` to the directory containing the source code (and this
+file), and then run:
+
+    autoreconf -iv
+
+If this runs successfully then you are ready to continue with one of
+the two installation methods below.
+
 Basic Installation via `Module::Build`
 --------------------------------------