fix: upload screenshot to github cdn

Author: vinkla

.gitattributes

@@ -4,6 +4,5 @@
 /.gitignore export-ignore
 /.prettierrc export-ignore
 /.travis.yml export-ignore
-/screenshot.png export-ignore
 /spec export-ignore
 /typings export-ignore

README.md

@@ -17,7 +17,7 @@ A replacement of the DataTip functionality from the original Atom-IDE / Nuclide
 
 3. Install `busy-signal` package in Atom, which is used for signaling background tasks.
 
-![A screenshot of your package](screenshot.png)
+![A screenshot of your package](https://user-images.githubusercontent.com/499192/54529587-95d64580-4980-11e9-827d-957cf48778ab.png)
 
 ## Contributing