Add development & testing instructions

[cpeel]

No functional changes to code, just documentation and skipping a test if the dependencies aren't there.

================================ test session starts =================================
platform linux -- Python 3.9.25, pytest-8.4.2, pluggy-1.6.0
rootdir: /mnt/hgfs/cpeel/git/libgutenberg
collected 23 items

libgutenberg/tests/test_cover.py .                                             [  4%]
libgutenberg/tests/test_dc.py ..............                                   [ 65%]
libgutenberg/tests/test_files.py s.                                            [ 73%]
libgutenberg/tests/test_logger.py ...                                          [ 86%]
libgutenberg/tests/test_media_types.py ..                                      [ 95%]
libgutenberg/tests/test_orm.py .                                               [100%]

================================== warnings summary ==================================
libgutenberg/Models.py:25
  /mnt/hgfs/cpeel/git/libgutenberg/libgutenberg/Models.py:25: MovedIn20Warning: The ``declarative_base()`` function is now available as sqlalchemy.orm.declarative_base(). (deprecated since: 2.0) (Background on SQLAlchemy 2.0 at: https://sqlalche.me/e/b8d9)
    Base = declarative_base()

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
===================== 22 passed, 1 skipped, 1 warning in 28.43s ======================

[cpeel]

If this Pipfile is only used for development I can move this to the non-dev dependency, let me know.