Add shutdown_signal to README

README.md

@@ -283,6 +283,7 @@ Just about everything in Gooey's overall look and feel can be customized by pass
 | show_time_remaining | Disable the time remaining text see [Elapsed / Remaining Time](#elapsed--remaining-time) |
 | hide_time_remaining_on_complete | Hide time remaining on complete screen see [Elapsed / Remaining Time](#elapsed--remaining-time) |
 | requires_shell | Controls whether or not the `shell` argument is used when invoking your program. [More info here](https://stackoverflow.com/questions/3172470/actual-meaning-of-shell-true-in-subprocess#3172488) |
+| shutdown_signal | Specifies the `signal` to send to the child process when the `stop` button is pressed. See [Gracefully Stopping](https://github.com/chriskiehl/Gooey/tree/master/docs) in the docs for more info. | 
 | navigation | Sets the "navigation" style of Gooey's top level window. <br>Options: <table> <thead> <tr><th>TABBED</th><th>SIDEBAR</th></tr></thead> <tbody> <tr> <td><img src="https://github.com/chriskiehl/GooeyImages/raw/images/readme-images/34464826-2a946ba2-ee47-11e7-92a4-4afeb49dc9ca.png" width="200" height="auto"></td><td><img src="https://github.com/chriskiehl/GooeyImages/raw/images/readme-images/34464847-9918fbb0-ee47-11e7-8d5f-0d42631c2bc0.png" width="200" height="auto"></td></tr></tbody></table>|
 | sidebar_title | <img src="https://github.com/chriskiehl/GooeyImages/raw/images/readme-images/34472159-1bfedbd0-ef10-11e7-8bc3-b6d69febb8c3.png" width="250" height="auto" align="right"> Controls the heading title above the SideBar's navigation pane. Defaults to: "Actions" |
 | show_sidebar | Show/Hide the sidebar in when navigation mode == `SIDEBAR` |