docs: add comprehensive README with contribution guide

Covers repository structure, server definition format, transport types,
input placeholder syntax, auth types, categories, naming conventions,
and step-by-step contribution workflow.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Mohammod Al Amin Ashik <maa.ashik00@gmail.com>

Author: its-mash