Updating the README with the Ubuntu Setup information for Reactotron

Summary: Issue reactotron#815 prevents reactotron to connect to the android emulator. Running adb reverse fixes the issue.

Test Plan: Running the adb reverse command fixed the Issue on Ubuntu 20.04.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, palys-swm, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1340