[gtk/matthiasc/for-master] Add a README in gtk/compose

[Matthias Clasen <matthiasc src gnome org>]

commit d4b7a78c54127a2bb7fd3f0eab06a3cc361953f1
Author: Matthias Clasen <mclasen redhat com>
Date:   Thu Sep 2 20:10:19 2021 -0400

    Add a README in gtk/compose

 gtk/compose/README.md | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)
---
diff --git a/gtk/compose/README.md b/gtk/compose/README.md
new file mode 100644
index 0000000000..ba86f77a59
--- /dev/null
+++ b/gtk/compose/README.md
@@ -0,0 +1,19 @@
+Compose data
+============
+
+GTK includes a copy of the X11 Compose file in a compact, precompiled
+form (the same format is used for caching custom Compose files in
+the users home directory).
+
+The X11 Compose file can be found here:
+
+  https://gitlab.freedesktop.org/xorg/lib/libx11/-/raw/master/nls/en_US.UTF-8/Compose.pre
+
+The tool to convert the data is build during the GTK build, from
+compose-parse.c in this directory. You run it like this:
+
+  cpp -DXCOMM='#' Compose.pre | sed -e 's/^ *#/#/' > Compose
+  compose-parse Compose sequences-little-endian sequences-big-endian chars gtkcomposedata.h
+
+The GTK build expects the resulting files to be in the source tree,
+in the gtk/cmompose directory.