Add "features" / "value prop" section to README

[bduffany]

IMO it would really help the project take off if the README had a "features" section. As a developer when I see a new tool, the number one question I want answered is "why should I use this?"

The "motivation" section currently leads with "you are right that there are a lot of web dashboards out there." But I actually didn't even know that there are a lot of Web dashboards out there! The README should not compare itself to other dashboards as the primary motivation, but instead focus on the features that it does provide and why people should use it.

Maybe this section could answer the following:

• [ ] Supports running multiple dev servers in parallel and seeing their terminal outputs in separate tabs?
• [ ] Supports sending keyboard commands to the running servers (e.g. Ctrl+C, or providing input on STDIN when needed)?
• [ ] Supports stopping/starting servers from the dashboard? (i.e. STOP/START button)
• [ ] Supports "preset" configurations/actions? (i.e. start all prod servers, start all dev servers)
• [ ] Supports having the dash4 configuration in its own package, to avoid polluting the other packages with dash4 config?
• [ ] Supports alerts (e.g. desktop notifications) when a server crashes?
• [ ] Supports some kind of indication that a server has experienced an error (i.e. logged to stderr)?

A link to a demo video could also be very helpful!