add README for performance microbenchmarks with usage instructions

Author: chandra-siri

tests/perf/microbenchmarks/README.md

@@ -0,0 +1,38 @@
+# Performance Microbenchmarks
+
+This directory contains performance microbenchmarks for the Python Storage client library.
+
+## Usage
+
+To run the benchmarks, use `pytest` with the `--benchmark-json` flag to specify an output file for the results.
+
+Example:
+```bash
+pytest --benchmark-json=output.json -vv -s tests/perf/microbenchmarks/reads/test_reads.py
+```
+
+### Running a Specific Test
+
+To run a single test, append `::` followed by the test name to the file path.
+
+Example:
+```bash
+pytest --benchmark-json=output.json -vv -s tests/perf/microbenchmarks/reads/test_reads.py::test_downloads_single_proc_single_coro
+```
+
+## Configuration
+
+The benchmarks are configured using `config.yaml` files located in the respective subdirectories (e.g., `reads/config.yaml`).
+
+## Overriding Buckets
+
+You can override the buckets used in the benchmarks by setting environment variables. Please refer to the specific benchmark implementation for the environment variable names.
+
+## Output
+
+The benchmarks produce a JSON file with the results. This file can be converted to a CSV file for easier analysis in spreadsheets using the provided `json_to_csv.py` script.
+
+Example:
+```bash
+python3 tests/perf/microbenchmarks/json_to_csv.py output.json
+```