desktop: document a number of environment variables

Change-Id: Ie33f8b2f8d2a65fbdb7f4f3caaeee8f0f046cba5
(cherry picked from commit de244205)

desktop/README.vars

+Environment variables in desktop:
+
+General
+-------
+
+DISPLAY - X11 display to use.
+OOO_DISABLE_RECOVERY - Disables the recovery dialog.
+OOO_EXIT_POST_STARTUP - Exit right after startup, for profiling purposes.
+SAL_DISABLE_USERMIGRATION - Disable automatic conversion of old user configurations.
+SAL_USE_VCLPLUGIN - Which VCL plugin to use instead of the auto-detected one.
+
+LibreOfficeKit
+--------------
+
+LOK_DEBUG - Draw a small red rectangle in the top left corner so that it's easy to see where a new tile begins.
+LOK_VIEW_CALLBACK - Use incomplete per-view callbacks instead of a single one.