README for each unit

[mcouplet]

For each unit, we should have a README that will show automatically and contains the following information:

• brief overview of the content
• reference to section in Green Book
• prerequisites for the unit (see Prerequisites  #1)
• problems to solve (increasing order of difficulty)

See unit 13 as an example.
Could each of you that created a unit write a README? I think it will be helpful for contestants.

[RobinJadoul]

I believe it is possible to just use relative links, such that github will resolve them itself, instead of hardcoding the github link

[mcouplet]

@RobinJadoul alright, thanks for the remark.

[vlecomte]

I've added a README for my units and a general README for the repo, tell me what you think.

In terms of unit READMEs, the only ones that are still missing afaik are the ones for units 3 and 19. We are also missing a Dutch translation for the description in unit 2.

[mcouplet]

@vlecomte Great, this repo starts to look very professional :)
I think we should translate the main README in French and Dutch, so that the younger ones feel welcome. I propose the very basic units (like 1,2,3,4,6) are completely translated (README + slides) and we leave the rest in English.