remark on thread numbers

Signed-off-by: Matteo Muraca <[email protected]>

README.md

@@ -17,7 +17,8 @@ provided in Plonky2's repository.
 To run the tests, run the command `cargo test --release`.
 The command `cargo test` works too, but in that case you may want
 to give your computer a couple of minutes to compute the test results.
-The flag `-- --nocapture` can be used to print the execution times for proof generation and verification.
+
+The option `-- --nocapture --test-threads=1` can be used to print the execution times for proof generation and verification. The threads number must be set as 1, as the default parallel execution may interfere with the timing.
 
 The single circuits can be tested by matching the test name with `sudoku`, `permutation` or `factorial`.