Adds Javadoc guidelines to CONTRIBUTING.md.
Adds Javadoc guidelines to CONTRIBUTING.md.

@a-veitch suggested the initial guidelines, and I edited them to fit into a
section in CONTRIBUTING.md.

dinooliva/instrumentation-java,bogdandrutu/instrumentation-java,census-instrumentation/opencensus-java,bogdandrutu/opencensus-java,sebright/opencensus-java,sebright/opencensus-java,bogdandrutu/opencensus-java,sebright/instrumentation-java,songy23/instrumentation-java,bogdandrutu/opencensus-java,dinooliva/instrumentation-java,census-instrumentation/opencensus-java,ubschmidt2/instrumentation-java,sebright/opencensus-java,songy23/instrumentation-java,census-instrumentation/opencensus-java,sebright/instrumentation-java,bogdandrutu/instrumentation-java,songy23/instrumentation-java,ubschmidt2/instrumentation-java