Link break about windows in error message.

I got this error. I opened https://github.com/sfackler/rust-openssl#windows mentioned in the error, but there is no documentation.

I can't find any documentation for Windows.

  --- stderr
  thread 'main' panicked at C:\Users\mikoto\.cargo\registry\src\index.crates.io-6f17d22bba15001f\openssl-sys-0.9.102\build\find_normal.rs:190:5:

  Could not find directory of OpenSSL installation, and this `-sys` crate cannot
  proceed without this knowledge. If OpenSSL is installed and this crate had
  trouble finding it,  you can set the `OPENSSL_DIR` environment variable for the
  compilation process.

  Make sure you also have the development packages of openssl installed.
  For example, `libssl-dev` on Ubuntu or `openssl-devel` on Fedora.

  If you're in a situation where you think the directory *should* be found
  automatically, please open a bug at https://github.com/sfackler/rust-openssl
  and include information about your system as well as this message.

  $HOST = x86_64-pc-windows-msvc
  $TARGET = x86_64-pc-windows-msvc
  openssl-sys = 0.9.102

  It looks like you're compiling for MSVC but we couldn't detect an OpenSSL
  installation. If there isn't one installed then you can try the rust-openssl
  README for more information about how to download precompiled binaries of
  OpenSSL:

  https://github.com/sfackler/rust-openssl#windows

  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

In source code:

https://github.com/sfackler/rust-openssl/blob/22ffa9add64d962782d993fae0cd9b348d3b43cd/openssl-sys/build/find_normal.rs#L184

sfackler / rust-openssl

Link break about windows in error message. #2259