README.md: Improve intro text

The first change syncs README.md with Y2R's one.
README.md: Improve text in the "Update ycpc" section

Syncs README.md with Y2R's one.
README.md: Use backslashes to split a long command

Syncs README.md with Y2R's one.
README.md: Add packages needed for testing to installation instructions
README.md: Fix small errors
README.md: Improve workflow description in the "Overview" section

The terminology and wording is now closer to terminology and wording
used in command descriptions.
README.md: Link the "Module metadata" section better
README.md: Add info about YaST module support & qualify few statements
Merge pull request #608 from yast/last-minute-readme-fixes-try-2

[Review] Request from '' @ 'yast/ycp-killer/last-minute-readme-fixes-try-2'