Update documentation

[crtahlin]

To the level to make it possible for a developer to install and use the modules (examples, etc.). This applies to the list below (possibly to some other parts not listed below, please go through all the modules and check something is not missing or wrong after all the changes):

• https://github.com/datafund/data-receipt
• all the npm modules

[tfius]

check https://github.com/datafund/datareceipt.js readme, data there could be merged or referenced

[crtahlin]

Don't know if relevant, but package.json in all npm modules should probably have MIT licence?
public/manifest.json should have some description, or maybe should be deleted if we don't need it?