Update README about installation within Docker.

[Dr10-TakeHiro]

I found Dockerfile in README don't install your plugin properly. (#21 looks like same trouble.)

Therefore, I revise README as follows:

• Add an installation of python module in Dockerfile.
• Add an notion about cron deamon in Jupyter Docker image.

[jovanni-hernandez]

@Dr10-TakeHiro Thank you so much for your contributions to the project!