populate: a README of its own, krops' linking to itkrops-2

pkgs/populate/README.md documents populate on its own: building it,
what it needs, its command line, the sources as JSON and in Nix with
its Nix library, the target directory, the source types, flake inputs
from sources, what it prints, and its tests.  The sections about the
target directory, the source types, and flake inputs from sources move
there from krops' README, which links to them from sections of the same
names, and from the first mention of populate.  docs/design.md keeps
how populate works.

krops' README describes krops' own tests, and CHANGELOG what populate
being a program of its own means for krops.

0 files changed, 0 insertions, 0 deletions