Simplify Example, Extend README

[gsauthof]

Since zap-logfmt registers itself as an encoder with zap, (arguably), it's even simpler to use than the current example in the README suggests, i.e. e.g.:

package main

import (
	"go.uber.org/zap"
	_ "github.com/jsternberg/zap-logfmt"
)

func main() {
	config := zap.NewProductionConfig()
	config.DisableCaller = true
	config.Encoding = "logfmt"
	logger, _ := config.Build()
	logger.Info("Hello World")
}

That means it's 5 lines shorter, doesn't require knowledge of zap's core API and doesn't change other zap defaults (i.e. logging to stdout instead of stderr and changing the logging severity to debug - although debug isn't used in the example).

It produces the same output as the current first example:

ts=1598901552.010505 level=info msg="Hello World"

Thus, perhaps you want include something like the above in your README.

Perhaps you also want to extend the README a bit:

1. Explicitly mention that this package registers itself with the "logfmt" key such that Config.Encoding can be used to switch to logfmt output. FWIW - as being new to zap and your package - this wasn't obvious to me.
2. Add some example output for the examples.