Mention mingw in README

### Windows

Install OpenSSL from [here][1]. Cargo will not be able to find OpenSSL if it's

On Windows, consider building with [mingw-w64](http://mingw-w64.org/).

Build script will try to find mingw in `PATH` environment variable to provide

Cargo with location where openssl libs from mingw-w64 package may be found.

If you followed guide [Building on Windows](https://github.com/rust-lang/rust#building-on-windows)

from rust repo, then you should have [MSYS2](http://msys2.github.io/) with

`mingw-w64-openssl` installed as part of `mingw-w64-x86_64-toolchain`

(or `mingw-w64-i686-toolchain`) package.

Alternatively, install OpenSSL from [here][1]. Cargo will not be able to find OpenSSL if it's

installed to the default location. You can either copy the `include/openssl`

directory, `libssl32.dll`, and `libeay32.dll` to locations that Cargo can find

or pass the location to Cargo via environment variables: