Update README.rst with more detail

Devcurmudgeonrequested to merge
devcurmudgeon/buildstream-ps-update-readme into master

@tristanvb @jjardon I've fixed in response to Tristan's comments. Couldn't figure out how to get my patch to land in the existing MR, though :/

Related with #103 (closed)

Edited by Javier Jardón

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