Add a section about the Checker Framework to CONTRIBUTING.md (fixes #918).
Add a section about the Checker Framework to CONTRIBUTING.md (fixes #918).

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