Add installation guideline in docs

[gabiudrescu]

Steps to reproduce:

• search on google „monofony”
• open documentation https://monofony.readthedocs.io/en/latest/ or deprecated monofony repo
• ask yourself how to install this

what happens:

• you are a bit confused and left with a bad feeling about this

what should happen:

• you should see in the first page from docs the installation guideline

if approved, I can provide PR to fix this.

[loic425]

Yes, we have to move the installation guide into the documentation.

[oliver-schulz]

@gabiudrescu also take a look at the sylius documentation. Basically monofony is sylius without the eCommerce taste.

@loic425 i really like this packages and was able to build an app in about a week.