x/exp/cmd/gorelease: add usage
x/exp/cmd/gorelease: add usage

This makes it a bit easier to figure out how to use, rather than inferring from
fairly long main text. :)

Change-Id: Ie7a0c1cf3b77db2537964a77077b7f4ca80c4a49
Reviewed-on: https://go-review.googlesource.com/c/exp/+/251458
Run-TryBot: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>

golang/exp,golang/exp,golang/exp