This moves the documentation out of "Basic Usage" and expands on some of
the caveats of this approach.

This fixes GH #97 by introducing an alternate version of find_packages
that works with PEP 420 namespace packages.

docs: fix developer guide

fix PEP 518 configuration

Add `build-system.requires` key to `pyproject.toml`:
- this is necessary with pip 10.0.1, as otherwise the defaults will be
  to require both `setuptools` and `wheel` (and we only need the later)
- a `pyproject.toml` with no `build-system.requires` key will be invalid
  and rejected by a future version of pip
  (see https://github.com/pypa/pip/issues/5416#issuecomment-399638608)

Drop `Source Code` section, as it contains an outdated guideline
with respect to documenting changes and its contents are already
covered by other sections.

Avoid resource warning from dev scripts installed editable

Avoid ` ResourceWarning: unclosed file <_io.TextIOWrapper name='whatever' mode='r' encoding='UTF-8'>` on dev scripts installed editable

Fix Town Crier Link

Apparently when specifying any doctest_optionflags, it's necessary to also indicate the defaults. Add ELLIPSIS to fix other failing tests.