Include information about development packages

Building the crate on a system without the development packages of openssl installed fails.
Issue #649 suggests to install those packages, which resolves the problem.

openssl-sys/build.rs

@@ -134,6 +134,9 @@ 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.