message: doc: add note about org.antlr.v4.gui.TestRig and composite grammars
diff: && 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
  ```
  + * note that for composite grammars (*Lexer.g4 + *Parser.g4) bout grammars should be agument of antlr4
  + and the name of grammar for org.antlr.v4.gui.TestRig is name without Parser/lexer.
  