Document the EXTRA_RUNTEST_OPTS variable

This variable was previously undocumented.

README.md

@@ -26,6 +26,14 @@ options to a nofib run using the `EXTRA_HC_OPTS` variable like this:
     make boot
     make EXTRA_HC_OPTS="-fllvm" >&1 | tee nofib-llvm-log
 
+Likewise, you can pass additional arguments (e.g. RTS flags) to the command
+itself by using the `EXTRA_RUNTEST_OPTS` variable like this:
+
+    make EXTRA_RUNTEST_OPTS="-- +RTS -A2M -RTS"
+
+The `--` here ensures that `runtest` doesn't attempt to interpret any of the
+given flags as its own.
+
 To compare the results of multiple runs, use the program in
 `../utils/nofib-analyse`, for example: