README.EXT: about string termination

* README.EXT (Appendix B): add RSTRING_PTR and RSTRING_LEN.
  and NULL-termination by StringValueCStr.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45627 b2dd03c8-39d4-4d8f-98ff-823fe69b080e