Add a screenshot

Author: nedtwigg

.github/screenshot_highlighting.png

@@ -5,9 +5,9 @@
 - "Syntax highlighting is the lie that enables us to see the truth."
 - "The meaning of life is to find your lost comment drafts. The purpose of life is to post them."
 
-TODO: screenshot of GitHub comment syntax highlighting
+![screenshot of syntax highlighting in the edit pane of a GitHub issue](.github/screenshot_highlighting.png)
 
-TODO: screenshot of comment draft storage and restoration
+TODO: screenshot of comment draft index
 
 If there's something you'd like to add or fix, see [CONTRIBUTING.md](CONTRIBUTING.md).