Add compiling instructions section to readme

[thunderbiscuit]

Just to make it clear on how to compile for personal use. Something like this maybe?

Compile Shellcaster Yourself

You can build the binaries for yourself by going through the following steps:

git clone https://github.com/jeff-hughes/shellcaster.git
cd shellcaster
cargo build --release
cd ./target/release/bin/
cp ./shellcaster /usr/local/bin/    # for Linux and MacOS

[jeff-hughes]

Good call! I've added some instructions to the readme.