rebar.config.sample: document require_*_vsn options

rebar.config.sample

@@ -5,6 +5,11 @@
 
 %% == Core ==
 
+%% Check required ERTS or OTP release version
+{require_erts_vsn, ".*"}.
+{require_otp_vsn, ".*"}.
+{require_min_otp_vsn, ".*"}.
+
 %% Additional library directories to add to the code path
 {lib_dirs, []}.