Create README.md

Author: benjchristensen

src/perf/java/rx/README.md

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+# JMH Benchmarks
+
+### Run All
+
+```
+./gradlew benchmarks
+```
+
+### Run Specific Class
+
+```
+./gradlew benchmarks '-Pjmh=.*OperatorMapPerf.*'
+```
+
+### Arguments
+
+Optionally pass arguments for custom execution. Example:
+
+```
+./gradlew benchmarks '-Pjmh=-f 1 -tu ns -bm avgt -wi 5 -i 5 -r 1 .*OperatorMapPerf.*'
+```
+
+To see all options:
+
+```
+./gradlew benchmarks '-Pjmh=-h'
+```