Add better description to README.md
Add better description to README.md

Add an expository paragraph to the README explaining the core use case
supported by the plugin. Also clarify that "runtime" refers to
Composer's runtime and not the runtime of the managed application.

Closes #52

Priyank57/composer-merge-plugin,codeandcountry/composer-merge-plugin,webflo/composer-merge-plugin,bd808/composer-merge-plugin,wikimedia/composer-merge-plugin