Adds guide: action on focus

[jonsamp]

Adds a guide: Call an Action After Focusing Screen

This is in response to this Help Wanted issue: #191

See the guide here: https://deploy-preview-330--react-navigation.netlify.com/docs/en/action-after-focusing-screen.html

[react-navigation-bot]

Deploy preview for react-navigation ready!

Built with commit 50d6871

https://deploy-preview-330--react-navigation.netlify.com

[brentvatne]

thanks @jonsamp, this looks great! a few notes:

• before merging we should create an identical file in the unversioned docs directory
• it may be worth mentioning that using the HOC will re-render the screen when isFocused changes unless you implement shouldComponentUpdate. so if you want to make an api call it may be better to use the listener instead, since that won't re-render until you do a setState after the api call completes.
• it may also be worth mentioning that another use case may be to pause some resource (eg: watching location, playing a video) when switching away from the screen

[jonsamp]

thanks @brentvatne for the feedback! i incorporated your feedback and also copied the markdown file to the docs/ directory. please let me know if you have more feedback!

[brentvatne]

looks great, thanks @jonsamp :)