Add a README in gtk/compose

diff --git a/gtk/compose/README.md b/gtk/compose/README.md
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+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.