crypto/ecdsa: add a package level example
crypto/ecdsa: add a package level example

Change-Id: I4063d5ec4ac45561b94472b528583be564981912
Reviewed-on: https://go-review.googlesource.com/120144
Run-TryBot: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Filippo Valsorda <4f40cda291c5f9634e1affd3db44947af61f705c@golang.org>

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