Add a README for the spine-js-client library

[dpikhulya]

A spine-js-client library would ideally need a README that would describe all of the things that a developer needs to know to get started using the client library, such as at least:

• How do we attach it to our application (we need a Webpack build currently)?
• The fact that we need to create an instance of BackendClient as a primary object for contacting the Spine backend.
  • Probably an example might be helpful here, e.g. here's how we are creating BackendClient in License Server web client: backend.js.
• How do we fetch data?
• How do we send commands?
• A reference(s) to Spine Event Engine docs that have a description of the related concepts.

This README could be available in both the downloaded npm artifact, and the respective npm page.

See the other libraries for examples of what it can look like:

• other client libraries: firebase, twitter;
• simple ones: uuid, isarray.