Add badges to README

[AntoineRondelet]

Let's improve the README by adding the CI badges (that we will be able to expend to "coverage" badges etc down the line)

[AntoineRondelet]

A few "build status" badges have been introduced in #337. While we may want to add more badges going forward (for e.g. to track to status of a test suite etc), I think this ticket can now be closed.