Some readme tweaks

## Building

rust-openssl depends on the OpenSSL runtime libraries version 1.0.1 or above.

Currently the libraries need to be present in the build environment before this

rust-openssl depends on OpenSSL version 1.0.1 or above, or LibreSSL. Both the

libraries and headers need to be present in the build environment before this

crate is compiled, and some instructions of how to do this are in the sections

below.

### OSX

Although OpenSSL 0.9.8 is preinstalled on OSX this library is being phased out

of OSX and this crate also does not support this version of OpenSSL. To use this

of OSX and this crate also does not support that version of OpenSSL. To use this

crate on OSX you'll need to install OpenSSL via some alternate means, typically

homebrew:

Homebrew:

```bash

brew install openssl

```

If Homebrew is installed to the default location of `/usr/local`, OpenSSL will be

automatically detected.

### Windows MSVC

On MSVC it's unfortunately not always a trivial process acquiring OpenSSL.