Update README.md install intructions

[akriger]

Updated README install instructions for android to include the following. The below is needed when building a production build, otherwise it will return error -27 because it cannot talk to the Socket Mobile Companion app.

Targeting Android 28+?

Apps that target Android 28+ must also enable cleartext traffic to localhost. If you do not already have a network security configuration file, you will need to create one.

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="..." >
  <application android:label="..."
      android:networkSecurityConfig="@xml/network_security_config"
      ...>
  </application>
</manifest>

res/xml/network_security_config.xml

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
  <base-config cleartextTrafficPermitted="false" />
  <domain-config cleartextTrafficPermitted="true">
    <domain includeSubdomains="false">localhost</domain>
    <domain includeSubdomains="false">127.0.0.1</domain>
  </domain-config>
</network-security-config>