Add documentation on client & server API package generation.

[edegenetais-nx]

We need to write the inforamtion about how to build and publish a new ps-api client or server API package (client/server API jar) down in this project's readme.

[edegenetais-nx]

@laura-murat : je te recommande cette tâche tant que tu as les informations sous la main et fraîches en mémoire.