[vte] emulation: Add some research notes

[Christian Persch <chpe src gnome org>]

commit a40bf76e8b6a6222e680cd4793b7042e2b052474
Author: Christian Persch <chpe src gnome org>
Date:   Sun Jan 31 23:10:29 2021 +0100

    emulation: Add some research notes

 src/vteseq.cc | 10 ++++++++++
 1 file changed, 10 insertions(+)
---
diff --git a/src/vteseq.cc b/src/vteseq.cc
index 9a04e9aa..41e78d53 100644
--- a/src/vteseq.cc
+++ b/src/vteseq.cc
@@ -7000,6 +7000,16 @@ Terminal::RLOGIN_MML(vte::parser::Sequence const& seq)
 {
         /*
          * RLOGIN_MML - RLogin music markup language
+         * Music macro language and Midi file support.
+         *
+         * The music macro language appears to be (a variant of,
+         * or based on) the Microsoft and/or Tandy BASIC MML, see
+         * [http://www.vgmpf.com/Wiki/index.php?title=Microsoft_BASIC_MML]
+         * and
+         * [http://www.vgmpf.com/Wiki/index.php?title=Tandy_BASIC_MML].
+         * for more information on them, and the RLogin source code; as
+         * well as [http://nanno.dip.jp/softlib/man/rlogin/ctrlcode.html#DCS]
+         * for this escape sequence's parameters.
          *
          * Probably not worth implementing.
          *