scalar: convert README.md into a technical design doc

scalar: convert README.md into a technical design doc

Adapt the content from 'contrib/scalar/README.md' into a design document in
'Documentation/technical/'. In addition to reformatting for asciidoc,
elaborate on the background, purpose, and design choices that went into
Scalar.

Most of this document will persist in the 'Documentation/technical/' after
Scalar has been moved out of 'contrib/' and into the root of Git. Until that
time, it will also contain a temporary "Roadmap" section detailing the
remaining series needed to finish the initial version of Scalar. The section
will be removed once Scalar is moved to the repo root, but in the meantime
serves as a guide for readers to keep up with progress on the feature.

Signed-off-by: Victoria Dye <vdye@github.com>
Acked-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>