Fix JS typo and add details in installation in README

[Liscare]

After using Peaks.js, I find 3 "errors" in JS snippets and I found the "Using Peaks.js in your own project" section not really clear for a beginner.

This PR is only about the README file:

• 3 typos in Javascript snippets
• Add one step in the user guide ("Using Peaks.js in your own project" section)
• Refactor the "Using Peaks.js in your own project" section

[chrisn]

Thank you!