Update developer-guide.md to include openssl
Update developer-guide.md to include openssl

In order to use Crypto on .NET Core (which NuGet needs when talking https), you need to install openssl on OSX.
jkotas/cli,gkhanna79/cli,stuartleeks/dotnet-cli,borgdylan/dotnet-cli,blackdwarf/cli,borgdylan/dotnet-cli,gkhanna79/cli,borgdylan/dotnet-cli,stuartleeks/dotnet-cli,stuartleeks/dotnet-cli,jkotas/cli,jkotas/cli,blackdwarf/cli,jkotas/cli,stuartleeks/dotnet-cli,jkotas/cli,blackdwarf/cli,gkhanna79/cli,gkhanna79/cli,gkhanna79/cli,borgdylan/dotnet-cli,stuartleeks/dotnet-cli,anurse/Cli,blackdwarf/cli,borgdylan/dotnet-cli