add SUPPORT.md file

- https://github.com/blog/2400-support-file-support

Author: etpinard

.github/SUPPORT.md

@@ -0,0 +1,6 @@
+Implementation help may be found on community.plot.ly (tagged
+[`plotly-js`](http://community.plot.ly/c/plotly-js)) or on Stack Overflow
+(tagged [`plotly`](https://stackoverflow.com/questions/tagged/plotly)).
+
+Direct developer email support can be purchased through a [Plotly Support
+Plan](https://support.plot.ly/libraries/javascript).