Add bench/README.

bench/README

+This is a regression test suite. It is NOT a performance test suite.
+
+The subdirectory good/ contains a series of test grammars which Menhir is
+supposed to accept. We test that the following commands:
+
+  menhir --only-preprocess
+  menhir --explain -lg 2 -la 2 -lc 2
+
+both succeed and produce the expected output. We do NOT test that the
+generated parsers behave correctly.
+
+The subdirectory bad/ contains a series of test grammars which Menhir is
+supposed to reject. We test that the following command:
+
+  menhir
+
+fails and produces the expected output.
+
+Some tests involve multiple files. By convention, if several files have the
+same name up to a numeric suffix, then they belong in a single group and
+should be fed together to Menhir.