docs(README.md) improve readability of the README

[d2s]

• Add main heading title to the document, describing the repository contents.
• Use right Markdown heading types instead of bold text style.
• Split long text block to list items.
• Add Markdown -syntax links pointing to mentioned groups.
• Add extra lines of whitespace for easier readability of the plain text file.

[d2s]

Preview of the updated structure can be seen at: https://github.com/d2s/open-api-definition/tree/readme-improvement

[Mygee]

"This branch has conflicts that must be resolved"