source/index.rst: Use the README as our intro

Javier Jardónrequested to merge
jjardon/doc_readme into master

So we keep the contents of the README and https://buildstream.gitlab.io/buildstream/ in sync

(I was about to add the coverage badge in https://buildstream.gitlab.io/buildstream/ but I think this is better, even more after !403 (merged) is merged)

Edited by Javier Jardón

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