add blurb

Author: terryjreedy

Misc/NEWS.d/next/Documentation/2018-05-19-15-59-29.bpo-33479.4cLlxo.rst

@@ -0,0 +1,4 @@
+Remove the unqualified claim that tkinter is threadsafe. It has not been
+true for several years and likely never was. An explanation of what is true
+may be added later, after more discussion, and possibly after patching
+_tkinter.c,