Of course you can develop the HTML / JS part of the App on any operating system (which should be 99% of your app). However compiling the App for iOS is only possible on a Mac computer with Xcode installed. There are some “non-free” cloud services available which can do the compiling job. For example: http://www.macincloud.com/. I’ve never used one of these services but I am sure you’ll find enough information about them on Google.