Add mapPadding to docs

[codypearce]

Is this a bug report?

Yes

Have you read the Installation Instructions?

Yes

Environment

https://github.com/react-community/react-native-maps/blob/master/docs/mapview.md

Steps to Reproduce

1. Go to https://github.com/react-community/react-native-maps/blob/master/docs/mapview.md

2. Ctrl + f "mapPadding", no results returned

3. PR that implemented mapPadding Add mapPadding property #1750

Expected Behavior

Map View should have a prop section for mapPadding

Actual Behavior

There is no such prop section in docs

Reproducible Demo

https://github.com/react-community/react-native-maps/blob/master/docs/mapview.md

PR that implemented mapPadding #1750

This is a very useful prop that will help a lot of people who are running into problems, like I have, with fitToSuppliedMarkers or fitToElements cutting markers and other things off.