Filer: Add information about key_filter to README. (Redmine issue: #6799)

warden_filer/README

@@ -86,6 +86,8 @@ JSON object, containing configuration. See also warden_filer.cfg as example.
       filter - filter fields (same as in Warden query, see Warden and Idea
             doc, possible keys: cat, nocat, group, nogroup, tag, notag),
             unmatched events get discarded and deleted
+      key_filter - regular expression for keys which should be omitted from
+            the sent events
       node - o information about detector to be prepended into event Node
             array (see Idea doc). Note that Warden server may require it to
             correspond with client registration