List Compose Unicode entry

Closes #286. nemo++ Relevant IRC conversation: http://irclog.perlgeek.de/perl6/2016-01-04#i_11828959

Author: zoffixznet

doc/Language/unicode_entry.pod

@@ -15,6 +15,22 @@ General information about entering unicode under various operating systems
 and environments can be found on the Wikipedia L<unicode input
 page|https://en.wikipedia.org/wiki/Unicode_input>.
 
+=head1 XCompose
+
+Xorg includes digraph support using a Compose key
+https://en.wikipedia.org/wiki/Compose_key#GNU.2FLinux .  
+The default of AltGr + Shift can be remapped to something easier such as
+Capslock. In GNOME2 2 and MATE this can be setup under
+Preferences → Keyboard → Layouts → Options → Position of Compose Key.  
+So, for example, to input »+« you could type CAPSLOCK > > + CAPSLOCK < <
+
+XCompose allows customising the digraph sequences using a .XCompose file and
+https://github.com/kragen/xcompose/blob/master/dotXCompose is an extremely
+complete one. In GNOME, XCompose was overridden and replaced with a
+hardcoded list, but it is possible to restore XCompose by setting
+GTK_IM_MODULE=xim in your environment. It might be necessary to install
+a xim bridge as well, such as uim-xim.
+
 =head1 Editors and shells
 
 =head2 Vim