Add nuance to outsource-bottom-up

[regularfry]

Previously the outsource-bottom-up.md guidance was tilted strongly towards promoting cloud functions at the expense of any other compute model, whereas the CCoE policy is more relaxed. The previous wording ignores difficulties inherent in the cloud function model which are worth expanding on so that the reasoning behind the recommendations is clearer.

Addresses #48, although whether it fixes it is up for debate.

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