Update readme

[f-f]

Description of the change

• heavily update the README following some very good guidelines on writing technical documentation: now documentation is split between parts with different needs/tone instead of being in a giant tutorial:
  • "Tutorial"
  • "Howto"
  • "Explanation"
  • "Troubleshooting"
• address documentation for several issues:
  • fix Issue with calling purescript functions from javascript #193
  • fix spago installed via npm does not seem to fully install #192
  • Yarn-installed Spago doesn't work on Windows #187 (issue will be kept open for further investigation)
  • fix Question about separating executables / tests from libraries #178, Document how to add dev-only dependencies #191
  • fix Provide a way to get dependency licenses #150
  • fix Publishing a library with Spago #142

[JordanMartinez]

@f-f Alex and I read through that very helpful post, but we also read through a number of other resources. I'm not sure whether you'd be interested in it, but we summarized our conclusions here

[f-f]

@JordanMartinez thanks, that's a nice read too! 👏