win,msi: warn that older io.js needs manual uninstall

Because the UpgradeCode changes with version 1.0.2, older versions won't
be uninstalled automatically. This patch puts a warning in the
installer.

PR: nodejs#431
Reviewed-by: Rod Vagg <rod@vagg.org>

tools/msvs/msi/WixUI_en-us.wxl

@@ -1,4 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
 <WixLocalization Culture="en-us" xmlns="http://schemas.microsoft.com/wix/2006/localization">
-    <String Id="InstallDirDlgDescription">Choose a custom location or click Next to install</String>
+    <String Id="WelcomeDlgDescription">The Setup Wizard will install [ProductName] on your computer.&#xD;&#xA;&#xD;&#xA;WARNING: if you're upgrading from io.js v1.0.0 or v1.0.1, you must first uninstall these versions manually.</String>
+    <String Id="InstallDirDlgDescription">Choose a custom location or click Next to install.</String>
 </WixLocalization>