Build & translation / Add all informations in the same guide.

[fxprunayre]

Related to #82

[jodygarnett]

This change (moving the instructions from README to manual) confused me and conflicts with plan in core-geonetwork to move developer instructions to README.md files

[fxprunayre]

conflicts with plan in core-geonetwork to move developer instructions to README.md files

I was not aware of this plan but indeed :) but if you think it makes more sense to have the dev doc in README.md files, no objection to revert this.
The idea during the doc sprint last November (https://github.com/geonetwork/core-geonetwork/wiki/Documentation-codesprint-November-2019) was to try to remove redundant information in different places (with sometimes different information) to limit the number of places to maintain.

[jodygarnett]

I am not PSC, just noticing when I went to update docs the different sections of the code are diverging in different ways. Can we coordinate on mailing list?