User introduction to UI

[sfeilmeier]

From openems-gui created by sfeilmeier : OpenEMS/openems-gui#14

Bug Report or Feature Request (mark with an x)

- [ ] bug report -> please search issues before submitting
- [x] feature request

Desired functionality.

The user should see a short introduction to the user interface on first access. For example: http://introjs.com/. It is necessary to explain to the user, that there is a control delay between reading from meter, controlling the inverter and again reading from meter.