Update README.extern with regard to c_string.

Remove any indications that Chapel strings and c_strings can be
interchanged, as it is not pertinent since we have mechanisms to avoid
doing so already.
Merge pull request #527 from sungeunchoi/test

Update README.extern with regard to c_string.

Remove any indications that Chapel strings and c_strings can be
interchanged, as it is not pertinent since we have mechanisms to avoid
doing so already.

[Reviewed by Mike]