Reorganize README.md

[kjlyon]

Most of the content is already there for my suggestions, it just needs to be organized and basic explanations expanded a little. README.dm Suggestions below:

Under Snap Pluginsync Utility header include 2-3 sentences for each point. This will give context to the tool when someone is first introduced to it.

• How does it work
• What does it add to the plugin
• Why do we use this tool

Suggested structure reorganization below (comments in italics):
Installation

Usage

• Running: start container via command ./pluginsync.sh, note about ssh keys needing to be setup for this to run
• Update Snap plugin repository
• Generate travis secret
• Plugin statistics: this section needs explanation about how to get statistics for just one plugin
• (Remove Large Tests section)

Under the hood

• What is pluginsync actually doing when you run it.
• What is set up, configured or what files are auto-generated when pluginsync is run.
• This section could mention something about how it sets up for large tests.

Configuration
- Keep what you currently have