Add documentation to make getting started with apicast easier

[andrewdavidmackenzie]

We wish to make it easier for people new to apicast (who maybe also new to nginx, openresty and lua) to get started, use it and add their customizations.

Here is a list of customization examples that we would like to have explained briefly how to do.

• Adding a new server with own server name (ssl, nginx config)
• Adding some nginx http block configuration (like resolver)
• Adding custom lua config to override some existing functions
• Adding a custom module (e.g. XC)

These could be short documents on each one, in the corresponding examples sub-folder or small sections in an overall "customizations" document in examples root, to accompany any code examples for the same in the examples folder

[vramosp]

For the 'adding a custom module' there is something already I think, not sure if enough or needs to be completed. See https://github.com/3scale/apicast/blob/0f70f5ec90522d4e20df7ace42aef5c64e8c4135/examples/custom-module/README.md