Version bump for 0.6b.0 release (open-telemetry#536)

Version bump for 0.6b.0 release (open-telemetry#536)

Changelog and version updates for 0.5b0 release (open-telemetry#498)

Co-authored-by: Chris Kleinknecht <libc@google.com>
Co-authored-by: Leighton Chen <lechen@microsoft.com>

Fix new ext READMEs (open-telemetry#444) (open-telemetry#461)

Version bump for 0.4a0 release (open-telemetry#442)

Fix PR number typo in changelog

Update changelogs for 0.2a0 release (open-telemetry#253)

Update README for alpha release (open-telemetry#189)