Add links to the change note with details [skip ci]

CHANGES.rst

@@ -6,11 +6,14 @@
 
 - Add support for Python 3.7 and drop support for Python 3.3.
 
-- Build the CFFI modules (used on PyPy or when PURE_PYTHON is set) at
-  installation or wheel building time when CFFI is available. This
-  replaces the deprecated way of building them at import time. If
-  binary wheels are distributed, it eliminates the need to have a
-  functioning C compiler to use PyPy. See `issue 75
+- Build the CFFI modules (used on PyPy or when PURE_PYTHON is set) `at
+  installation or wheel building time
+  <https://cffi.readthedocs.io/en/latest/cdef.html#ffibuilder-set-source-preparing-out-of-line-modules>`_
+  when CFFI is available. This replaces `the deprecated way
+  <https://cffi.readthedocs.io/en/latest/overview.html#abi-versus-api>`_
+  of building them at import time. If binary wheels are distributed,
+  it eliminates the need to have a functioning C compiler to use PyPy.
+  See `issue 75
   <https://github.com/zopefoundation/persistent/issues/75>`_.