Add acknowledgements to docs

[giovannipizzi]

(for release 0.12.3)

[coveralls]

Coverage remained the same at 48.028% when pulling 6e677fe on giovannipizzi:add_ack_docs_012 into ea079fb on aiidateam:release_v0.12.3.

[sphuber]

@giovannipizzi the tests fail for the docs, because the header style you chose for the acknowledgments section is not consistent with those above. The django tests failing is just due to SSH problems

[giovannipizzi]

Thanks, I just noticed it.
I copied from 1.x but probably there we changed the headers. I think that there will be conflicts when merging, but should be easy to solve.