perf: document sqlx-bench and pool fairness benchmark

Author: abonander

sqlx-bench/README.md

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+SQLx Self-Benchmarks
+====================
+
+This Cargo project implements various benchmarks for SQLx using
+[Criterion](https://crates.io/crates/criterion).
+
+### Available Benchmarks
+
+* Group `pg_pool`: benchmarks `sqlx::Pool` against a PostgreSQL server.
+    * `DATABASE_URL` must be set (or in `.env`) pointing to a PostgreSQL server. 
+    It should preferably be running on the same machine as the benchmarks to reduce latency. 
+    * The `postgres` feature must be enabled for this benchmark to run.
+    * Benchmarks:
+        * `bench_pgpool_acquire`: benchmarks `Pool::acquire()` when many concurrent tasks are also using
+        the pool, with or without the pool being fair. Concurrently to the benchmark iteration
+        function calling and blocking on `Pool::acquire()`, a varying number of background tasks are
+        also calling `acquire()` and holding the acquired connection for 500µs each before releasing
+        it back to the pool. The pool is created with `.min_size(50).max_size(50)` so we shouldn't
+        be measuring anything but the actual overhead of `Pool`'s bookeeping.
+
+### Running
+
+You must choose a runtime to execute the benchmarks on; the feature flags are the same as the `sqlx` crate:
+
+```bash
+cargo bench --features runtime-tokio
+cargo bench --features runtime-async-std
+```
+
+When complete, the benchmark results will be in `target/criterion/`. 
+Open `target/criterion/report/index.html` or pick one of the benchmark subfolders and open
+`report/index.html` there to view the results.
+
+Benchmark Results
+-------
+
+If you want to share the results here, please follow the format below.
+
+* [2020/07/01: `pg_pool` benchmark added to test pool fairness changes](results/2020-07-01-bench_pgpool_acquire/REPORT.md)