docs: Mention including widths in split_command

And remove trailing spaces

README.md

@@ -180,6 +180,8 @@ Pass a table to the setup call with your configuration options.
     -- `position` will not be considered if `split_command` is non-nil.
     -- This should be a valid vim command used for opening the split for the
     -- outline window. Eg, 'rightbelow vsplit'.
+    -- Width can be included (with will override the width setting below):
+    -- Eg, `topleft 20vsp` to prevent a flash of windows when resizing.
     split_command = nil,
 
     -- Percentage or integer of columns