implemented the idea presented by Sander for dwm target

Makefile

@@ -21,7 +21,10 @@ options:
 	@echo CC $<
 	@${CC} -c ${CFLAGS} $<
 
-${OBJ}: dwm.h
+${OBJ}: dwm.h config.h
+
+config.h:
+	cp config.default.h config.h
 
 dwm: ${OBJ}
 	@echo LD $@

README

@@ -13,10 +13,9 @@ Installation
 Edit config.mk to match your local setup (dwm is installed into
 the /usr/local namespace by default).
 
-Afterwards enter the following commands to build and install dwm (if
+Afterwards enter the following command to build and install dwm (if
 necessary as root):
 
-    cp config.default.h config.h
     make clean install