Readme: Update library stage to Beta. (#1290)
Readme: Update library stage to Beta. (#1290)

* Update library stage to Beta.

* Add specification about versioning.

* Remove Alpha and add more description on Beta and GA.

sebright/opencensus-java,songy23/instrumentation-java,songy23/instrumentation-java,bogdandrutu/opencensus-java,songy23/instrumentation-java,sebright/instrumentation-java,census-instrumentation/opencensus-java,sebright/instrumentation-java,sebright/opencensus-java,census-instrumentation/opencensus-java,bogdandrutu/opencensus-java,census-instrumentation/opencensus-java,bogdandrutu/instrumentation-java,bogdandrutu/instrumentation-java,sebright/opencensus-java,bogdandrutu/opencensus-java