Suggestion: add Build instructions

[jannone]

I cloned to repository and ran yarn (same as npm install)
Then I opened the project in XCode.
When selecting Product -> Build, I get this error:

/react-native-region-monitor/ios/INVRegionMonitor.h:1:9: 'React/RCTBridgeModule.h' file not found

It seems the code requires additional steps before building.
Maybe it is an obvious process, but since it's my first time playing with a react-native plugin codebase, I would pretty much welcome some build instructions.