doc: document support for global environment variables

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/config.md

@@ -73,6 +73,36 @@ document [Finit Services](service.md).
 > examples in the `contrib/` directory, which can be used as a base.
 
 
+### Environment Variables
+
+In Finit v4.3 support for setting environment variables in `finit.conf`,
+and any `*.conf`, was added.  It is worth noting that these are global
+and *shared with all* services -- the only way to have service-local
+environment is detailed in the next section.
+
+The syntax for global environment variables is straight forward:
+
+    foo=bar
+    baz="qux"
+
+On reload of .conf files, all tracked environment variables are cleared
+so if `foo=bar` is removed from `finit.conf`, or any `finit.d/*.conf`
+file, it will no longer be used by Finit or any new (!) started
+run/tasks or services.  The environment of already started processes can
+not be changed.
+
+The only variables reset to sane defaults on .conf reload are:
+
+    PATH=_PATH_STDPATH
+    SHELL=_PATH_BSHELL
+    LOGNAME=root
+    USER=root
+
+It is entirely possible to override these as well from the .conf files,
+but be careful.  Changing SHELL changes the behavior of `system()` and a
+lot of other commands as well.
+
+
 Service Environment
 -------------------