patch 8.1.0311: filtering entries in a quickfix list is not easy

Problem:    Filtering entries in a quickfix list is not easy.
Solution:   Add the cfilter plugin. (Yegappan Lakshmanan)
diff --git a/runtime/doc/quickfix.txt b/runtime/doc/quickfix.txt
index 1399d8b..211fe96 100644
--- a/runtime/doc/quickfix.txt
+++ b/runtime/doc/quickfix.txt
@@ -1551,6 +1551,22 @@
 recognized as a command separator.  The backslash before each space is
 required for the set command.
 
+						*cfilter-plugin*
+If you have too many matching messages, you can use the cfilter plugin to
+reduce the number of entries.  Load the plugin with: >
+   packadd cfilter
+
+Then you can use these command: >
+   :Cfilter[!] {pat}
+   :Lfilter[!] {pat}
+
+:Cfilter creates a new quickfix list from entries matching {pat} in the
+current quickfix list. Both the file name and the text of the entries are
+matched against {pat}. If ! is supplied, then entries not matching {pat} are
+used.
+
+:Lfilter does the same as :Cfilter but operates on the current location list.
+
 =============================================================================
 8. The directory stack				*quickfix-directory-stack*