Update README.md to address old bison problem on macOS.

README.md

@@ -46,6 +46,10 @@ To run the tests, execute the following from the project's root directory.
 
     $ rake test
 
+Note: `bison` bundled with MacOS is too old to compile `mruby`.
+Try `brew install bison` and follow the instuction shown to update
+the `$PATH` to compile `mruby`.
+
 ## Building documentation
 
 There are two sets of documentation in mruby: the mruby API (generated by yard) and C API (Doxygen)