test/codegen: document -all_codegen option in README
test/codegen: document -all_codegen option in README

It is useful to know about the -all_codegen option for running
codegen tests for all platforms. I was puzzling that some codegen
test was not failing on my local machine or on trybot, until I
found this option.

Change-Id: I062cf4d73f6a6c9ebc2258195779d2dab21bc36d
Reviewed-on: https://go-review.googlesource.com/c/go/+/192101
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
Run-TryBot: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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