Add examples on how to use the ConfigBuilder

[AdheipSingh]

Thanks for this library.
I am new to rust and was playing around with the examples to get started.

Just encountered this warning on compilation

warning: use of deprecated associated function `config::Config::merge`: please use 'ConfigBuilder' instead
 --> src/main.rs:7:10
  |
7 |         .merge(config::File::with_name("Settings")).unwrap()
  |          ^^^^^
  |
  = note: `#[warn(deprecated)]` on by default

warning: use of deprecated associated function `config::Config::merge`: please use 'ConfigBuilder' instead
  --> src/main.rs:10:10
   |
10 |         .merge(config::Environment::with_prefix("APP")).unwrap();
   |          ^^^^^

warning: 2 warnings emitted

    Finished dev [unoptimized + debuginfo] target(s) in 0.06s
     Running `target/debug/simple`
{"priority": "32", "debug": "false", "key": "189rjfadoisfj8923fjio"}

Just wondering if the example can be updated to configBuilder. An explanation on the usage of configbuilder vs merge will helpful too.

[matthiasbeyer]

This (examples not using deprecated stuff anymore) is WIP in #223 !

An example would definitively be nice, so I'll leave this open.

[Davidprogram2]

what does this mean please?

[matthiasbeyer]

I just checked, the documentation for the ConfigBuilder type already includes an example, so there's nothing to do here actually. I'll close this now.