Update README to remove 'hugo server' command

[zbayoff]

The README mentions in the Development section to use the npm run build which will run hugo server, but this is not the case, looking at the package.json scripts. hugo server isn't used at all, I believe since the switch from Gulp to Webpack. May want to remove this as it causes confusion.

Aside, I wonder if there may be a way to allow the use of hugo server for those who edit markdown files, and don't touch the js or css. The js and css would already need to be compiled and located elsewhere from the dist folder. This would make development much faster for teams editing content.