Change extension for README and WORKFLOW

Change .asciidoc to more standard and shorter .adoc.

This potentially breaks some links to workflow in master from outside but in general is has to happen to be consistent with other projects and future changes in them. On notice @jschlehofer as he linked workflow to various places at least on forum.

Requested in https://gitlab.labs.nic.cz/turris/turris-os-packages/merge_requests/156