DOC: Add information about in-process kernels to What's New.

docs/source/whatsnew/development.txt

@@ -12,6 +12,17 b' especially intenting/deindenting blocks that is now bound to Ctrl+] and ctr+['
   ``_render_traceback_()`` method which returns a list of strings, each
   containing one line of the traceback.
 
+In-process kernels
+------------------
+
+The Python-language frontends, particularly the Qt console, may now communicate
+with in-process kernels, in addition to the traditional out-of-process
+kernels. An in-process kernel permits direct access to the kernel namespace,
+which is necessary in some applications. It should be understood, however, that
+the in-process kernel is not robust to bad user input and will block the main
+(GUI) thread while executing. Developers must decide on a case-by-case basis
+whether this tradeoff is appropriate for their application.
+
 Backwards incompatible changes
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