Mention some of the motivation for this package

Try to preemptively answer hypothetical questions like Why can't I just

```
    class Proxy(object):
        def __init__(self, obj):
            self.__obj = obj
        def __getattr__(self, a):
            return getattr(self.__obj, a)
        def __setattr__(self, a, v):
            setattr(self.__obj, a, v)
```

README.rst

-
 ===========================
 Generic Transparent Proxies
 ===========================
@@ -8,6 +7,11 @@ wrappers around another object, intervening in the apparent behavior of
 the wrapped object only when necessary to apply the policy (e.g., access
 checking, location brokering, etc.) for which the proxy is responsible.
 
+zope.proxy is implemented via a C extension module, which lets it do things
+like lie about its own ``__class__`` that are difficult in pure Python (and
+were completely impossible before metaclasses).  It also proxies all the
+internal slots (such as `__int__`/`__str__`/`__add__`).
+
 Editorial note:
 
    Unfortunately, we don't have separate documentation for `zope.proxy`