nsis: added a note about installer icons in README.txt

This fixes a few issues introduced in commit 3495936:

-  Deleted the icons directory
-  Duplicate files in the icons.zip archive have been removed
-  Added a note about installer icons in README.txt

closes: #18469

Signed-off-by: RestorerZ <restorer@mail2k.ru>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Filelist

@@ -642,7 +642,6 @@ SRC_DOS =	\
 		nsis/auxiliary.nsh \
 		nsis/gvim.nsi \
 		nsis/gvim_version.nsh \
-		nsis/icons/README.txt \
 		nsis/Makefile \
 		nsis/Make_mvc.mak \
 		nsis/README.txt \

nsis/README.txt

@@ -108,3 +108,5 @@ Variables and their values available for building the installer (not mandatory):
 			    Example "X=OutFile MyVim.exe;XPMode on"
     "MKNSIS=<dir>"	— the directory where the "makensis.exe" program is
 			    located.
+
+Note: for icons used in the installer, see the README.txt in the icons.zip