stash/docker/compiler/README.md

Modified from https://github.com/bep/dockerfiles/tree/master/ci-goreleaser

When the dockerfile is changed, the version number should be incremented in the Makefile and the new version tag should be pushed to docker hub. The scripts/cross-compile.sh script should also be updated to use the new version number tag, and .travis.yml needs to be updated to pull the correct image tag.