Fill readmes with content and proper headlines

[haesleinhuepf]

We have a lot of readme.md files around that have not been filled with content properly yet. That's why our TOC looks like this at the moment.

It would be cool if we could check the headlines of our sections and write at least one introductory sentence in all readme.md files.

Thanks!

[haesleinhuepf]

It might make sense to write explanatory sentences such as "In this section we will to X which is good for Y" X can be linked to a wikipedia-article for example explaining what it is.

[zoccoler]

An update on this: day5 needs update on the readme file.

[haesleinhuepf]

Please also check if the links to the slides work, e.g. in Dimensionality Reduction and Clustering.
Thanks!

[jo-mueller]

Should be in order now