Improve README documentation

Summary:
Previously, README contained nothing but a link to userbase. A separate HACKING.md
file had non-technical developer documentation, but nothing technical to guide
people in the right direction.

This patch consolidates all information in the README file and adds technical
information about how to develop Dolphin.

BUG: 408967

Test Plan: Just read it :)

Reviewers: #documentation, #dolphin, yurchor

Reviewed By: #documentation, yurchor

Subscribers: yurchor, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D22025