Include instructions to compile from source

[whoisjeremylam]

As a Golang programmer
I want to compile plasma-cli from source
So that I can interact with the OMG Network without downloading scary binaries from the internet

[0x234]

If a user has set their $GOPATH correctly they can execute a go build from the root of this repo directly. We can add a SHA256 on the releases page so users can verify they're using a build we've created.