Added link to gif software for ubuntu (#5912)

For recording screen.

Author: chan750

docs/pull-requests.md

@@ -13,7 +13,7 @@
 
 Include screenshots and animated GIFs in your pull request whenever possible.
 
-**Recording GIFs** There are many great tools for recording a GIF. On a mac, we recommend [recordit](http://recordit.co/), which is a free lightweight app.
+**Recording GIFs** There are many great tools for recording a GIF. On a mac, we recommend [recordit](http://recordit.co/), which is a free lightweight app. If you are using Ubuntu, you can also try using [Peek](https://github.com/phw/peek).
 
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