[perf] Add some instructions for building/running benchmarks.

perf/README.md

@@ -0,0 +1,24 @@
+Benchmarks are implemented using [Google Benchmark](https://github.com/google/benchmark).
+
+To build the benchmarks in this directory you need to set the benchmark
+option while configuring the build with meson:
+
+```
+meson configure build -Dbenchmark=enabled
+```
+
+Then build the benchmark binaries with ninja:
+
+```
+ninja -Cbuild perf/benchmark-set
+```
+
+Finally, to run one of the benchmarks:
+
+```
+./build/perf/benchmark-set
+```
+
+It's possible to filter the benchmarks being run and customize the output
+via flags to the benchmark binary. See the
+[Google Benchmark User Guide](https://github.com/google/benchmark/blob/main/docs/user_guide.md#user-guide) for more details.