Added documentation for programmatically switching tabs

[ppsreejith]

Programmatically switching tabs was not documented and was pretty hard to find so I thought this could help future readers :)

[chirag04]

you can use navigator.performAction?

[ppsreejith]

I meant if you don't have access to this.props.navigation.

As said in the readme, this is primarily for the case where you want to change tabs from, say redux-saga or redux-thunk, programmatically where you don't have access to this.props.navigation.

[jasonmerino]

This documentation would be super handy to have published. It was kind of difficult to figure out how to use the NavigationActions.jumpToTab action.

@chirag04 I also didn't have access to this.props.navigation because I'm doing some routing in our app in a redux action file rather than a component file.