README: clarify build step

Help those unfamiliar with cabal to install shellcheck.
Merge pull request #12 from mcandre/master

README: clarify build step
Use exit status and stderr properly in terminal tool
Merge branch 'master' of github.com:koalaman/shellcheck
Fixed parser bug where }> wasn't recognized as Rbrace
Fixed parser accepting spaces after here doc token
Stable version 0.2.0
gitignore