Add more getting started infos in README.md

[KikiManjaro]

Feel free to add modifications to what i've written :) you know the project better than me

[multun]

@KikiManjaro just so you know, we have not forgotten this PR, we'd just like to add some more stuff to it

[multun]

We're making a documentation website which already contains most of what's in there. I'll change this PR to add a link to this documentation once it's in a satisfying state

https://dgexsolutions.github.io/osrd-website/fr/developers/

[multun]

Time has passed, and a lot of things have changed (including the README). I don't think it makes sense to merge anymore