Improve Readme (suggestions)

[Painatalman]

The current README is pretty clean and straightforward, but I'd like to make a few suggestions to make it more navigable and accessible:

• add a "prerequisites" section detailing software requirements. Each entry could be complemented with a link and version":

  
  ## Prerequisites

• Node.js LTS 12.16.2+
• NPM 6+ (already included in Node.js)
• Now CLI (optional, for local development)

• create subsections for steps to create a production build, to run in production, and to connect with Google Calendar. This way, it's easier to share and to navigate between page sections. Ex.:

  ## Build and running on localhost
  To test the app locally or to make a production build, you must first **install or update the project's dependencies**
  ...
  ### Create a Production Build
  After [installing and updating dependencies](#Building-and-running-on-localhost), run
  ...
  ### Running the app locally
  In addition to [installing and updating dependencies](#Building-and-running-on-localhost), you must also **have [Now CLI]()** to run the app locally. To **install it**, run...
  ...
  ### (optional) Test app locally with events from your google calendar
  ...

• Add links to external resources, like Now CLI, Google Calendar API, and [Simple API Access key]. This allows user to check on what they are and how to use them (for example, I don't really know what a Simple API Access key is)
• Add a contribution section, detailing what branch should be cloned and to which branch the PR should be done. If it becomes considerably large, we could make this a separate file, as well.

If everyone agrees, I'd be delighted to open a PR with my proposals.