Update README.markdown

[dhritzkiv]

I hope you don't mind that I updated parts of the README. Being new to Z-Wave and OpenZWave, I had a hard time following along (both this guide and the OpenZWave installation steps) and getting set up, so hopefully the following changes will make it clearer to others hoping to use this plugin.

Changes to the README:

• Fix configuration snippet to use commands instead of classes
• Reformat config snippet to be a bit more legible
• Change "gateway" to "controller" when talking about the physical device. Left references to "gateway" when referring to the abstract Z-Wave node type.
• Added another known/support Z-Wave controller
• Added installation instructions for OpenZWave for typical setups (apt/apk; from source on linux/raspberry pi)
• Added installation instructions if using homebridge-config-ui-x
• Fix some typos
• Wrapped certain terms in inline code blocks
• Removed some superfluous characters
• Move Homebridge to the Requirements section