Document compilation features

We have multiple compilation features that should be documented for new contributors.

changed milestone to %Horizon

added C-doc label

mentioned in merge request !255 (merged)

mentioned in merge request !256 (merged)

mentioned in commit 98dfa0a4

mentioned in commit 758af2e7

closed with commit 1e3c4043

closed with merge request !256 (merged)

mentioned in commit 1e3c4043

changed milestone to %client-0.8.0