Add some documentation about the globals in use.

doc/globals.md

@@ -0,0 +1,25 @@
+All the globals created by evoGUI are written into the `global.evogui` table,
+whose top-level keys are all player names. Thus, all evoGUI globals are
+player-specific.
+
+Globals were first used in evoGUI v0.3.0 -- earlier versions will not have
+these, but v0.3.0 and later will create them as needed.
+
+Currently, we use the following (prefix with `global.evogui[player_name]` to
+access):
+
+    - `version`: the version of evoGUI that created these settings. Primary use
+    is to trigger the recreation of the evoGUI main interface when doing an
+    up/downgrade.
+
+    - `always_visible`: a `LuaTable` whose keys are names of value sensors and
+    whose values are either true or nil. Used to determine whether a value
+    sensor should be shown to a specific player in the _always visible_ pane.
+
+    - `in_popup`: a `LuaTable` whose keys are names of value sensors. Used
+    analogously to `always_visible` for sensors that should be shown in popup
+    (i.e., when the main interface is expanded).
+
+    - `popup_open`: a `bool` that states whether the main interface is expanded
+    or not (i.e., whether `in_popup` sensors are visible or not). Changing this
+    unexpectedly may leave the GUI in a weird state.