add A mapping to maximize/restore tree window size

this commit is a modified patch from Guillaume Duranceau

doc/NERD_tree.txt

@@ -239,6 +239,7 @@ F.......Toggle whether files are displayed.......................|NERDTree-F|
 B.......Toggle whether the bookmark table is displayed...........|NERDTree-B|
 
 q.......Close the NERDTree window................................|NERDTree-q|
+A.......Zoom (maximize/minimize) the NERDTree window.............|NERDTree-A|
 ?.......Toggle the display of the quick help.....................|NERDTree-?|
 
 ------------------------------------------------------------------------------
@@ -546,6 +547,14 @@ Applies to: no restrictions.
 
 Closes the NERDtree window.
 
+------------------------------------------------------------------------------
+                                                                  *NERDTree-A*
+Default key: A
+Map option: NERDTreeMapToggleZoom
+Applies to: no restrictions.
+
+Maximize (zoom) and minimize the NERDtree window.
+
 ------------------------------------------------------------------------------
                                                                   *NERDTree-?*
 Default key: ?
@@ -1002,6 +1011,8 @@ Next release:
       NERDTreeQuitOnOpen didnt play nicely, thanks to Curtis Harvey.
     - fix a bug where the script ignored directories whose name ended in a dot,
       thanks to Aggelos Orfanakos for the patch.
+    - added a mapping to maximize/restore the size of nerd tree window, thanks
+      to Guillaume Duranceau for the patch. See :help NERDTree-z for details.
 
 3.1.1
     - fix a bug where a non-listed no-name buffer was getting created every
@@ -1094,6 +1105,7 @@ just downloaded pr0n instead.
     Alf Mikula
     Lucas S. Buchala
     Curtis Harvey
+    Guillaume Duranceau
 
 ==============================================================================
 8. License                                                   *NERDTreeLicense*