README.md: Simplify the introduction

All deleted information will be introduced later in a new section.
README.md: Improve step 2 in the installation a bit

There is no need to mention yast-core in the header + the second
paragraph lacked an obvious connection to the first one.
README.md: Move & rename the "Resulting Directory Structure" section

This section should be at the end because it is a reference really. The
new name ("New YaST Module Structure") also better reflects its
contents.
README.md: Add the "Overview" section

This section is needed to get a high-level idea about how YCP Killer
works and to understand the directory terminology.
README.md: Add Makefile.am creation step into the workflow overview
Merge pull request #372 from yast/readme-improvements

[Review] Request from '' @ 'yast/ycp-killer/readme-improvements'