Replace .gitkeep files with useful explanation of folder purpose

[katrinleinweber]

As discussed in #234. make serve is not impacted in any way that I notice. Is the initial text in the ReadMe.md files factually correct?

[katrinleinweber]

The build error does not seem to be related to these changes:

In library(package, lib.loc = lib.loc, character.only = TRUE, logical.return = TRUE,  :
  there is no package called ‘requirements’

[fmichonneau]

The relevant build error is:

Error in i.p(...) : 
  (converted from warning) 'lib = "/usr/local/lib/R/site-library"' is not writable"

it is triggered because the Travis file is not set up for R (only Python), and hence the environment variables and library aren't set up to allow package installation.

[katrinleinweber]

OK, thanks for explaining. Please feel free to hide my above comment about the resulting error.

Let's focus on the suggested changes again.

[katrinleinweber]

I found a possibly even cleaner way to avoid (almost) empty folders:

What do you think @rgaiacs & @fmichonneau? Since the makefiles of both lesson-example and styles have the same commit history, I guess such commits should be PR'ed towards the styles repo.

Also: Happy new year, and all the best for 2019 :-)