Add changelog entry for deprecation of pattern functions

changelog/pattern-deprecate.dd

@@ -0,0 +1,10 @@
+Several functions in `std.string` have been deprecated
+
+The functions $(REF inPattern, std, string), $(REF countchars, std, string),
+$(REF removechars, std, string), $(REF squeeze, std, string), and
+$(REF munch, std, string), have all been deprecated. These functions are
+obsolete, as their functionality is better covered by the functions in
+$(MREF std, regex) and $(MREF std, algorithm). They will be removed from
+$(MREF std, string) on May 2018.
+
+If you still need to use these, please see $(LINK2 https://github.com/dlang/undeaD, undeaD).