Added build info feedback in help

- `$I` prints the Grbl build info and version number. NOTE: `$I=xxx`
stores an additional 30 character string into EEPROM, which will be
printed with the build info the next time it’s run. This is for
identification purposes for users that have more than one system using
Grbl.

report.c

@@ -144,6 +144,7 @@ void report_grbl_help() {
   printPgmString(PSTR("$$ (view Grbl settings)\r\n"
                       "$# (view # parameters)\r\n"
                       "$G (view parser state)\r\n"
+                      "$I (view build info)\r\n"
                       "$N (view startup blocks)\r\n"
                       "$x=value (save Grbl setting)\r\n"
                       "$Nx=line (save startup block)\r\n"