[WP] (i20_gremlin_example): explain Makefile usage

src/green/gremlin1/examples/README

@@ -3,6 +3,25 @@ this is <README>
 toy example for gremlin
 -----------------------
 
+Some procedures for the toy example are defined in the Makefile. They can be
+run be issuing make with the appropriate target name. For example the command
+
+  make layer.pdf
+
+will create the target file layer.pdf which is discussed below. Any of the
+files being discussed but not present in the repository most likely is a make
+target. To watch the commands being executed for a given target, but without
+executing them actually run the command make with the '-n' flag like in
+
+  make -n layer.pdf
+
+----------------------------------------------------------------------------
+
+Standard targets are:
+
+clean
+  purges the directory, removes all temporary and secondary files
+
 ============================================================================
 
 layer.pdf