Create RELEASE-NOTES for 1.0.1

[viktorklang]

https://github.com/reactive-streams/reactive-streams-jvm/blob/master/RELEASE-NOTES.md

Including:

PublisherVerification:

• public void untested_spec305_cancelMustNotSynchronouslyPerformHeavyCompuatation() throws Exception {

• public void untested_spec305_cancelMustNotSynchronouslyPerformHeavyComputation() throws Exception {

[seratch]

Is the release date decided?

[viktorklang]

@seratch Not 100%—I'd like to get it out before end of June. I think we could do that, there's a bit of PR reviews / merging / testing to be had first tho.

[ktoso]

This can be closed I think?
Notes were merged with #379

[viktorklang]

@ktoso Aye.