Add beginner friendly install and usage instructions to readme.

[THEGOLDENPRO]

As the title says we must add some install and usage instructions to the readme, however they must be well written and obvious as many beginner developers will be reading and relying on theses instructions.

Ideally they should install snekilings with pip, create a folder and sneki init to create the exercises folder then run sneki start to begin working on the exercises and etc.