add directions for using the pprof feature on linux (#77)

how_to_profile.md

@@ -23,6 +23,21 @@ installed for it.
    analyzed.
 9. Select the areas of the diagnostics session when your benchmark is running to analyze.
 
+## Linux
+
+With the "pprof" feature enabled, the Criterion benchmarking library can be used
+to automatically output profiles with flamegraphs for each benchmark. Simply run
+the desired benchmarks while passing `--profile-time=10` or similar to the
+benchmark binary. (Passing this argument probably requires specifying the
+"bench" benchmark specifically, as other test binaries that cargo thinks might
+have benchmarks in them do not understand the flag.)
+
+Example command: `cargo bench --bench bench --no-default-features --features fooencoder,pprof -- 'minecraft_savedata/fooencoder/serialize' --profile-time=10`
+
+This will run the "minecraft_savedata/fooencoder/serialize" benchmark and
+profile it, then save a flamegraph of the profile results as a `.svg` file to
+`target/criterion/minecraft_savedata_fooencoder/serialize/profile/flamegraph.svg`.
+
 ## Other platforms
 
 Know how to profile these benchmarks on other platforms? Open a pull request!