Update documentation for __NSConstantString(_tag) removal (#32)

Author: iMichka

docs/history.rst

@@ -29,13 +29,19 @@ Version 1.7.3 (not yet released)
    CastXML. GCCXML did not recognize the typedef here. Fetching these
    declarations can now be done with: .typedef("foo") or .class_("foo").
 
-6. Remove unittest2 dependency for testing with python 2.6
+6. Add support for the future llvm 3.9. As in release v1.7.1, new structs and
+   typedefs are now exposed by llvm, which broke pyplusplus.
+   In this case these are ```__NSConstantString_tag``` and ```__NSConstantString```.
+   The two declarations are now hidden from the declarations tree, but can still
+   be generated by using the ``config.flags = ["f1"]`` option.
 
-7. Testing: test with std::tr1 unordered containers for gcc >= 4.4.7 and castxml
+7. Remove unittest2 dependency for testing with python 2.6
 
-8. Cosmetic fix for generator name printed to stdout when launching unit tests
+8. Testing: test with std::tr1 unordered containers for gcc >= 4.4.7 and castxml
 
-9. Fix simple typo in example.py comment
+9. Cosmetic fix for generator name printed to stdout when launching unit tests
+
+10. Fix simple typo in example.py comment
 
 Thanks to the following people for their contribution to this release:
 Mark Moll, Ashish Sadanandan, Mark Oates