Updates TWA Documentation to include Splash Screen (#7747)
Updates TWA Documentation to include Splash Screen (#7747)

* Updates TWA Documentation to include Splash Screen

- Adds a section to using-twa.md that adds the steps necessary
 to integrate Splash Screens on Chrome 75.

* Fixes text and wraps lines

* Improve TWA Documentation for Splash Screens

- Adds note that using a Vector Drawable is an alternative.
- Changes example to use a 320x320 image.
- Fixes typos

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