docs: added performance benchmarking doc

doc/contributing/benchmark-tests.md

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+
+# Performance Benchmark Testing Guide
+
+Benchmark tests are intended to measure performance of small units of code. 
+
+It is recommended that operations that have a high impact on the performance of the SDK (or potential for) are accompanied by a benchmark test. This helps end-users understand the performance trend over time, and it also helps maintainers catch performance regressions.
+
+Benchmark tests are run automatically with every release, and the results are available at https://open-telemetry.github.io/opentelemetry-js/benchmark.
+
+## Running benchmark tests
+
+Performance benchmark tests can be run from the root for all modules or from a single module directory only for that module:
+
+``` bash
+# benchmark all modules
+npm run test:bench
+
+# benchmark a single module
+cd packages/opentelemetry-sdk-trace-base
+npm run test:bench
+```
+
+### Adding a benchmark test
+
+Unlike unit tests, benchmark tests should be written in plain JavaScript (not Typescript).
+
+* Add a new test file in folder `test/performance/benchmark`.
+
+Use the following as a template:
+
+``` javascript
+const Benchmark = require('benchmark');
+
+const suite = new Benchmark.Suite();
+
+suite.on('cycle', event => {
+  console.log(String(event.target));
+});
+
+suite.add('new benchmark test', function() {
+	// write code to test ...
+});
+
+suite.run();
+```
+
+### Automatically running benchmark tests
+
+If you want your test to run automatically with every release (to track trend over time), register the new test file by requiring it in `test/performance/benchmark/index.js`.
+
+Add the `test:bench` script in package.json, if the module does not contain.
+
+``` json
+"test:bench": "node test/performance/benchmark/index.js"
+```