patch 8.1.0713: images for NSIS take up too much space

Problem:    Images for NSIS take up too much space.
Solution:   Put the images in a zip file.
diff --git a/nsis/README.txt b/nsis/README.txt
index f5348e9..6ecc579 100644
--- a/nsis/README.txt
+++ b/nsis/README.txt
@@ -67,6 +67,11 @@
 Install NSIS if you didn't do that already.
 Also install UPX, if you want a compressed file.
 
+Unpack the images:
+	cd nsis
+	unzip icons.zip
+
 To build then, enter:
 
+	cd nsis
 	makensis gvim.nsi