[App] [skip ci] add info for expression lexer generation

src/App/ExpressionParser.l

@@ -24,6 +24,13 @@
 
 /* Lexer for the FreeCAD Expression language   */
 
+/* use this file to generate the file 'lex.ExpressionParser.c' using the program flex
+ * the command for this operation is:
+ * flex --outfile=lex.ExpressionParser.c ExpressionParser.l
+ * (flex for Windows is available here:
+ *  https://sourceforge.net/projects/winflexbison/
+ *  (you must then change 'flex' to 'win_flex' in the command)) */
+
 /* This disables inclusion of unistd.h, which is not available under Visual C++
  * on Win32. The C++ scanner uses STL streams instead. */
 #define YY_NO_UNISTD_H