Added notes in readme

My rationale is here: 
`If a lockfile already exists, pub get uses the versions of dependencies locked in it if possible. If a dependency isn’t locked, pub will get the latest version of that dependency that satisfies all the version constraints. This is the primary difference between pub get and pub upgrade, which always tries to get the latest versions of all dependencies.` And I want the project to be always bleeding edge right now (because dependencies change fast right now. When they stabilize we can keep `pub get`