Update blend2bam to v0.6

Bump version to 0.4.2

Bump version to 0.4.1

README: Remove Read the Docs link

Merge branch 'gltf2'

Makefile: Add zip target

This builds a zip for release that can be installed by Blender.

Docs: Remove "Using an Existing Project" sub-section stub

This can be added back in when there is actual content for this
sub-section.