Add README outlining the bootstrap process

This outlines an initial version of the bootstrap workflow with all packages in the initial bootstrap set being bootstrapped from their respective pkgbases.

In archlinux/python-bootstrap!5 (merged) we are updating the respective submodules for python-bootstrap to support the current versions in the repository.

Changes to the respective packages are done in:

• python-build!1 (merged)
• python-flit-core!1 (merged)
• python-installer!1 (merged)
• python-packaging!1 (merged)
• python-pyproject-hooks!1 (merged)
• python-setuptools!2 (merged)
• python-wheel!1 (merged)