Create overview documentation for developers

[petringo]

Write an overview of the technical architecture for new developers getting familiar with the code.

Describe basic concepts, design decisions and principles, conventions.

Prefer

• high level over details
• brief description over verbose
• lists and diagrams over written sentences and paragraphs

Target audience is a developer of this library getting familiar with it.

Write the document as GitHub Flavored Markdown file under docs directory at root of the project tree.