Update README.md announcing v2.0.0-RC1 (#236)
Update README.md announcing v2.0.0-RC1 (#236)

Add links to v2.0.0-RC1 and upgrade documentation, and request for more testers.
markszabo/IRremoteESP8266,markszabo/IRremoteESP8266,markszabo/IRremoteESP8266,markszabo/IRremoteESP8266