docs(readme): clarify contribution guidelines and licensing

[fvsch]

Partial rewrite of the README.md with:

• Add main heading and fix heading hierarchy.
• Language tweaks (use for example instead of e.g., use instead of utilize, etc.)
• Naming convention: I used the phrase “starter templates” consistently instead of just “starters” because that’s what we’re using in the StackBlitz dashboard. Looking at https://developer.stackblitz.com we are using “starter projects” (most common), “project starters”, and there is one instance of “base template” and one of “starter templates”. Personally I’m not 100% convinced “starter templates” work for us, because we already use the word “template” with a different meaning in our SDK (for the project.template option).
• Add Contributing section.
• Add Licensing section.

All those choses are “best guess” efforts, and I’m open to any and all changes.

[sulco]

I think the starter templates are good enough for now:)

Great improvements across the board, thank you!