Update README

Author: cytopia

README.md

@@ -1,3 +1,23 @@
 # Makefiles
 
 Common Makefiles for Devilbox ecosystem.
+
+
+## Usage in main Makefile
+
+```makefile
+ifneq (,)
+.error This Makefile requires GNU Make.
+endif
+
+# Ensure additional Makefiles are present
+MAKEFILES = Makefile.docker Makefile.lint
+$(MAKEFILES): URL=https://raw.githubusercontent.com/devilbox/makefiles/master/$(@)
+$(MAKEFILES):
+	@if ! (curl --fail -sS -o $(@) $(URL) || wget -O $(@) $(URL)); then \
+		echo "Error, curl or wget required."; \
+		echo "Exiting."; \
+		false; \
+	fi
+include $(MAKEFILES)
+```