fmt: add a package-level example illustrating basic formats

fmt: add a package-level example illustrating basic formats

There is much left out here—the space of possibilities is very
large—but this example shows all that most programmers will need
to know for most printing problems.

Update #27554.

Change-Id: Ib6ae651d5c3720cf7fe1a05ffd0859a5b56a9157
Reviewed-on: https://go-review.googlesource.com/136616
Reviewed-by: Ian Lance Taylor <iant@golang.org>