Write "getting started" guide
This should go in https://buildstream.gitlab.io/buildstream/, probably as part of the installation guide
This should cover:
- what it's for in a couple of sentencess.
- how to install it (via a link if it's complex)
- how to do 'hello-world' within 10 minutes, with words people with no bst experience can understand
- where to dig deeper
Edited by Javier Jardón