Improve documents

[eitsupi]

• Add README to each directory.
  • Add description of the files that are automatically generated in each directory.
  • Slightly modified the scripts to ignore README.md in each directory, and removed the .gitkeep that was no longer needed due to the addition of the README.md.
• Minor changes to README.md in the project root directory.
  • Remove the size column that used to display the microbadger.com badge.(Remove the size column from the tables rocker#462)
  • Modify the Dockerfile example
    • 3.6.3 -> 4.0.0
    • r-ver -> rstudio
    • Only install Shiny server.

[cboettig]

Yes yes this is great!

@noamross want to take a quick look over this and merge?

[noamross]

Thanks for this @eitsupi, a great leap forward in documentation! I made one tweak to a legacy typo.