General fixes to README.md
This mentions dependencies for multiple distros, changes the path to ~/kde/src, and adds a few niceties from https://invent.kde.org/ashark/kde-builder/-/merge_requests/5.
This does not include the alternative installation proposed in the linked MR, it's just some QoL fixes.
To see how it looks in rendered Markdown: https://invent.kde.org/ashark/kde-builder/-/blob/2bb3365c1f586e2095a1b65175d2995b88620d07/README.md
Edited by Thiago Sueto