Add comment on how to write benchmarking tests

CONTRIBUTING.md

@@ -72,6 +72,25 @@ Performance progression of benchmarks for packages distributed by OpenTelemetry
 
 Running the `tox` tests also runs the performance tests if any are available. Benchmarking tests are done with `pytest-benchmark` and they output a table with results to the console.
 
+To write benchmarks, simply use the [pytest benchmark fixture](https://pytest-benchmark.readthedocs.io/en/latest/usage.html#usage) like the following:
+
+```python
+def test_simple_start_span(benchmark):
+    def benchmark_start_as_current_span(span_name, attribute_num):
+        span = tracer.start_span(
+            span_name,
+            attributes={"count": attribute_num},
+        )
+        span.end()
+
+    benchmark(benchmark_start_as_current_span, "benchmarkedSpan", 42)
+```
+
+Make sure the test file is under the `tests/performance/benchmarks/` folder of
+the package it is benchmarking and further has a path that corresponds to the
+file in the package it is testing. Make sure that the file name begins with
+`test_benchmark_`. (e.g. `opentelemetry-sdk/tests/performance/benchmarks/propagation/test_benchmark_b3_format.py`)
+
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