Add simple script to get going on mac

substrate/getgoing.sh

+/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
+brew install openssl cmake
+curl https://sh.rustup.rs -sSf | sh
+source ~/.cargo/env
+cargo install --git https://github.com/paritytech/substrate subkey
+cargo install --git https://github.com/paritytech/substrate substrate