Add caveat on different DPI scaling on Windows/Linux

See #714 and wkhtmltopdf/wkhtmltopdf#2184

README.md

@@ -392,6 +392,8 @@ However, the wicked_pdf_* helpers will use file:/// paths for assets when using
 
 If one image from your HTML cannot be found (relative or wrong path for ie), others images with right paths **may not** be displayed in the output PDF as well (it seems to be an issue with wkhtmltopdf).
 
+wkhtmltopdf may render at different resolutions on different platforms. For example, Linux prints at 75 dpi (native for WebKit) while on Windows it's at the desktop's DPI (which is normally 96 dpi). [Use `:zoom => 0.78125`](https://github.com/wkhtmltopdf/wkhtmltopdf/issues/2184) (75/96) to match Linux rendering to Windows.
+
 ### Inspiration
 
 You may have noticed: this plugin is heavily inspired by the PrinceXML plugin [princely](http://github.com/mbleigh/princely/tree/master).  PrinceXML's cost was prohibitive for me. So, with a little help from some friends (thanks [jqr](http://github.com/jqr)), I tracked down wkhtmltopdf, and here we are.