Add detail about VimuxOpenExtraArgs option to docs

doc/vimux.txt

@@ -362,5 +362,17 @@ redefine this if you're using something like tmate.
 
 Default: "tmux"
 
+------------------------------------------------------------------------------
+                                                              *VimuxOpenExtraArgs*
+2.8 g:VimuxOpenExtraArgs~
+
+Allows addtional arguments to be passed to the tmux command that opens the
+runner. Make sure that the arguments specified are valid depending on whether
+you're using panes or windows, and your version of tmux.
+
+  let g:VimuxOpenExtraArgs = "-c #{pane_current_path}"
+
+Default: "tmux"
+
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