Document how to deal with openssl and other often-missing external dependencies

[strct]

On debian cargo install cargo-c fails with the error:

  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-unknown-linux-gnu
  $TARGET = x86_64-unknown-linux-gnu
  openssl-sys = 0.9.58


  It looks like you're compiling on Linux and also targeting Linux. Currently this
  requires the `pkg-config` utility to find OpenSSL but unfortunately `pkg-config`
  could not be found. If you have OpenSSL installed you can likely fix this by
  installing `pkg-config`.

While the error message is quite descriptive and helps to install the required packages, it might be nice to include the install instructions in the README.md.
Required packages are described here: sfackler/rust-openssl#855 (comment)

[lu-zero]

It is a good idea, if you could come up with a patch for the readme I'd land it.

[lu-zero]

I updated the README to give some more information, let me know if you think it is enough.

[strct]

Thank you for writing it. Looks good to me.