compress/gzip: add example of compressing reader
compress/gzip: add example of compressing reader

For #51092

Change-Id: If0a233651ac75f113569ddfffd056084f6092564
Reviewed-on: https://go-review.googlesource.com/c/go/+/389514
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Joseph Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@digital-static.net>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,golang/go,golang/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,golang/go,golang/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go