Improve Composer Documentation

[mostertb]

This PR attempts to improve the documentation for using Composer in the README.md to suggest a more idiomatic approach in the hopes that it will help users avoid problems.

Specifically:

• installing composer using any of the official guides (or the Windows Installer) will help users with misconfigured PHP environments
• the use of composer init and composer require (instead of directly editing the composer.json avoids syntax errors while ensuring a valid set of version requirements.

[msftclas]

All CLA requirements met.