New release checklist

1. check copyright dates in doc sections in MQTTAsync.h and MQTTClient.h

2. update version number (major.minor.patch) in version files

3. check binary backward compatibility

4. merge develop branch into master

5. generate docs (make html) and update the docs folder

6. push all updates and make sure the builds work

7. tag the release, assuming all the previous steps have worked

8. update the release with the build artifacts

9. update the Paho website - and any other online docs