README: add icon with appveyor status
#+OPTIONS: ^:nil
[[https://travis-ci.org/Dushistov/rust_swig][https://travis-ci.org/Dushistov/rust_swig.svg?branch=master]]
[[https://github.com/Dushistov/rust_swig/blob/master/LICENSE][https://img.shields.io/badge/license-BSD-green.svg]]
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+[[https://ci.appveyor.com/api/projects/status/db4rs7f96iba4bt8/branch/master?svg=true]]
* Getting started
Suppose you have the following Rust code:
