cmd/gorelease: add testdata/README.md, explaining test format
cmd/gorelease: add testdata/README.md, explaining test format

Change-Id: I81cb0486b57796adfe735fdbd186c3f1431377a6
Reviewed-on: https://go-review.googlesource.com/c/exp/+/251821
Run-TryBot: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/exp,golang/exp,golang/exp