Add basic project documentation and example usage

Written and edited by @CAM-Gerlach (OP was initially empty):

• Add/move general information currently contained in __init__ docstring to the README for visibility
• Clarify that QtPy follows PyQt5 conventions, and link Qt and PyQt5 API docs
• Add examples of using QtPy in place of the specific bindings
• List/describe the methods/helpers specific to QtPy (listed in #61 )
  • from qtpy.QtCore import Signal, Slot
  • QFileDialog helpers
  • qtpy.QtCore.__version__
  • Others?
• Document the QtPy-specific environment variables (FORCE_QT_API, etc)
• Describe the main package-level constants (PYQT5, PYSIDE2, etc)
• Clarify the benefits of using QtPy with regard to cross-Qt-version migration, and link success stories (described in #62 )
  • git-cola/git-cola#232
  • Glue
• Clarify Qt version compatibility in the readme

[CAM-Gerlach]

As another low-hanging fruit item, the __init__.py contains a substantial amount of general information and examples in its docstring that could be moved to or included in the README to be much more visible, with a bit of cleanup.

[CAM-Gerlach]

Thanks, opened as PR #279