Update README.md

Updated your install instructions to use the recommended `composer require` command which avoids manually editing the json file and also runs install/update automatically for the new requirement, making it all happen in a single step. It also automatically finds the proper latest version.

I also added instructions for using dev-master, but i highly recommend tagging releases often. Feel free to ask me questions about this and the how and why it works.
Merge pull request #22 from rdohms/patch-1

Update README.md