add benchmarking

BENCHMARKING.md

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+# Benchmarking
+
+This program started out life as a fairly simple Go program that just runs everything in a single thread.
+I'm rewriting it in Rust for learning, but also because I think Rayon might let me make this problem parallelizable!
+
+Here's an initial benchmark on a `/usr/share/dict/words` of 235,886 lines:
+
+```
+$ hyperfine './result/bin/similar-sort benchmark < /usr/share/dict/words'
+Benchmark #1: ./result/bin/similar-sort benchmark < /usr/share/dict/words
+  Time (mean ± σ):     307.4 ms ±   6.0 ms    [User: 273.5 ms, System: 75.5 ms]
+  Range (min … max):   298.3 ms … 317.2 ms    10 runs
+```
+
+Let's see if we can get any faster than that!