Added instructions to first fetch submodules before installing.

Makefile

@@ -18,6 +18,7 @@ all:
 	@echo "       make uninstall"
 
 install:
+	@test -f gitflow-shFlags || (echo "Run 'git submodule init && git submodule update' first." ; exit 1 )
 	install -d -m 0755 $(GIT_EXEC_PATH)
 	install -m 0755 $(EXEC_FILES) $(GIT_EXEC_PATH)
 	install -m 0644 $(SCRIPT_FILES) $(GIT_EXEC_PATH)

README.mdown

@@ -12,7 +12,12 @@ blog post").
 
 Installing git-flow
 -------------------
-To install `git-flow` as a real `git` subcommand, run:
+After downloading the sources from Github, also fetch the submodules:
+
+	$ git submodule init
+	$ git submodule update
+
+Then, you can install `git-flow`, using:
 
 	$ sudo make install