Improve README / Guide for beginners

[mcm1957]

• README states that one should edit package,json and enter devDependency manually when user wants to perform a local installation.
  Installation should (in my oppinion) better be performed using npm instal ... command. At least there mjust be an advice, that adding to package.json doe not perform the real installation.

• symbol dev-server
  When performing a local installation and adding a script named dev-server ths command 'dev-server' as it is used at all exampled will not work. Either one muist use npm run dev-server or (at linux) some alias must be set. This should be describe so that novice users can start withpu much system admin knowledge.