Add installation note on how to install getopt.
Thanks Alexander Groß.
diff --git a/README.mdown b/README.mdown
index 6120337..804efc7 100644
--- a/README.mdown
+++ b/README.mdown
@@ -35,6 +35,12 @@
Or simply point your `PATH` environment variable to your git-flow checkout
directory.
+*Installation note:*
+git-flow depends on the availability of the command line utility `getopt`,
+which may not be available in your Unix/Linux environment. Please use your
+favorite package manager to install `getopt`. For Cygwin, install the
+`util-linux` package to get `getopt`.
+
Integration with your shell
---------------------------