Post extra step of yarn to set up android development environment in CONTRIBUTING

[RcleydsonR]

Description:

It is necessary to run the yarn start command in another terminal before the yarn android command to have the application running in an android development environment, this step is not included in the contributing document: "Setting up a development environment"

Environment Information:

It isn't necessary, but it's important to point out that this is a necessary step to run in an android environment.

Expected behavior:

After following the tutorial from the contributing guide, don't expect to face any problems running the android development environment.

Actual behavior:

Command hangs on 'Starting JS server...'

Additional context:

When I was following the tutorial to upload the development environment, I faced a problem when trying to run in android emulator because the tutorial only suggests using the yarn android command, not specifying the need to run the yarn start command to start the metro which is the javascript bundler, and is needed to run the application.

[dnlsilva]

It is not necessary to run yarn start before running yarn android.
Your terminal probably doesn't have "access" to open another terminal.

After running yarn android the other terminal opens automatically.

Probably related:
facebook/react-native#28807
https://stackoverflow.com/questions/52777413/reactnative-metro-bundler-not-starting-automatically