docs: mention HYPERFINE_ITERATION in README

README.md

@@ -87,6 +87,12 @@ option:
 hyperfine -L compiler gcc,clang '{compiler} -O2 main.cpp'
 ```
 
+If you need a unique value for each individual run of a benchmark command, hyperfine also exposes
+the zero-based `$HYPERFINE_ITERATION` environment variable inside the benchmarked command itself:
+```sh
+hyperfine 'my-command > output-${HYPERFINE_ITERATION}.log'
+```
+
 ### Intermediate shell
 
 By default, commands are executed using a predefined shell (`/bin/sh` on Unix, `cmd.exe` on Windows).