Add npm script to start development server and update README.

[lmeijvogel]

This PR changes the documentation package.json and README to simplify the Hugo development server startup.

[CLAassistant]

All committers have signed the CLA.

[MarkvanMents]

@lmeijvogel
Thanks for this Lennaert. This makes the command much simpler.
I will discuss with the team whether this is the best name for the command to build and serve the site, but great that you have pointed me at an npm function which I wasn't aware of.
I might keep the other instructions as well, in case someone wants to change the hugo parameters (to build production, for example) but this is a great improvement for 99% of the cases of everyday use.

[MarkvanMents]

Hi @lmeijvogel
Thanks for the PR, it makes life much easier!
I've left npm start as a script, so you can still use that, but added npm run build and documented that as it is more descriptive for users who are not used to npm conventions.
Thanks for improving the user experience for local builds.
Mark