Update warning text

src/output/warnings.rs

@@ -17,7 +17,10 @@ impl fmt::Display for Warnings {
         match *self {
             Warnings::FastExecutionTime => write!(
                 f,
-                "Command took less than {:.0} ms to complete. Results might be inaccurate.",
+                "Command took less than {:.0} ms to complete. Note that the results might be \
+                inaccurate because hyperfine can not calibrate the shell startup time much \
+                more precise than this limit. You can try to use the `-N`/`--shell=none` \
+                option to disable the shell completely.",
                 MIN_EXECUTION_TIME * 1e3
             ),
             Warnings::NonZeroExitCode => write!(f, "Ignoring non-zero exit code."),