Update windows install instructions for JSDuck 4.0.

which includes Ruby interpreter and all dependencies bundled in a

single .exe file.  Grab it from the [download page][].

If you are brave enough: [try out JSDuck 4.0 beta.][beta]

Alternatively you can install through rubygems, but you need to do

some additional tweaks. First go and [download][libs download]

therubyracer gem and v8 lib that stereobooster has built for

windows. You need to install this special rubyracer version instead of

the one from rubygems:

    > gem install therubyracer-0.11.0beta1-x86-mingw32.gem

To make it actually work you need `v8.dll` somewhere in your

system. Extract the `lib_v8.3.11.9.zip` take the `v8.dll` inside it

and place into the `bin` directory of your Ruby installation (other

dirs that are on your PATH can work too, but I've found this to be the

most sensible place to put it). Now you're ready to install JSDuck:

    > gem install jsduck

[download page]: https://github.com/senchalabs/jsduck/downloads

[beta]: https://github.com/senchalabs/jsduck/wiki/4.0-beta

[libs download]: https://github.com/stereobooster/therubyracer/downloads

Usage

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