Provides introductory information about repo

Since Fuel 6.0 has Pluggable architecture feature,
fuel-plugins repo serves to create plugins.

This commit adds some introductory information and links into
README.md file.

Change-Id: Iddacaa04d0db892fc0fee4140020001551c97d18
Co-Authored-By: Evgeny Li <eli@mirantis.com>
Merge "Provides introductory information about repo"