message: doc: note about org.antlr.v4.gui.TestRig
diff: * to forece regeneration of generated files delete cmake file which
  tells cmake that the files were generated e.g. `rm _skbuild/linux-x86_64-3.7/cmake-build/src/vhdlConvertor/vhdlParsers_GENERATED_SRC`
  +* antlr4 gui - visualize a parse tree
  + ```
  + CLASSPATH=/usr/share/java/stringtemplate4.jar:/usr/share/java/antlr4.jar:/usr/share/java/antlr4-runtime.jar:/usr/share/java/antlr3-runtime.jar/:/usr/share/java/treelayout.jar
  + mkdir gen && cd gen
  + antlr4 ../vhdl.g4 && mv ../*.java ../*.tokens ../*.interp . \
  + && javac -cp $CLASSPATH vhdl*.java \
  + && java -cp $CLASSPATH:$PWD "org.antlr.v4.gui.TestRig" vhdl design_file -gui ../../tests/vhdl/mux.vhd -help -diagnostics -trace -tokens
  + ```
  