Add ZIP_STATIC for Windows static library compilation, and add a
paragraph in README explaining it.

README

diff --git a/README b/README
index 756a282..de9171f 100644
--- a/README
+++ b/README
@@ -17,6 +17,9 @@
 with most operating systems nowadays, or you can get it at
 	http://www.gzip.org/zlib/
 
+  When using a static Windows library, you need to define ZIP_STATIC
+when compiling packages using libzip.
+
   If you make a binary distribution, please include a pointer to the
 distribution site:
 	http://www.nih.at/libzip/