Issues with setup instructions in README

[hndmrsh]

The installation instructions in the README are incomplete. I encountered the following issues when trying to add Plunge to my project:

• The repositories block is missing from the buildscript section
• The kotlinx-serialization dependency requires a Kotlin version > 1.3.20
• The kotlinx-serialization repository is missing for the plugin
• The line Uri.parse(getIntent().getData()) should just be intent.data (as getData() returns a Uri already)

[jamiesanson]

The repositories block and Uri parsing is an easy fix. IMO we should remove kotlinx-serialization and replace it with something available in JCenter. Moshi is likely the smallest drop-in replacement for this. I'll whip up another issue for this.