doc: clarify that contributing isn't just for code
doc: clarify that contributing isn't just for code

Updates #33323

Change-Id: I4b3076469f3db8ab0fe888edcf0471f17dc199e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/202823
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

akutz/go,golang/go,akutz/go,nawawi/go,akutz/go,nawawi/go,cloudflare/go,akutz/go,CAFxX/go,golang/go,CAFxX/go,golang/go,akutz/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,golang/go,akutz/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,golang/go,akutz/go,akutz/go