v0.5.1 — contributing, security and conduct docs; no change to the packages

The package code is the same as 0.5.0. The repository is ready for outside
contributors.

npm i @sheetpane/grid@0.5.1

Before you upgrade from 0.5.0: nothing to change.

Added

• The repository: CONTRIBUTING.md, SECURITY.md and CODE_OF_CONDUCT.md. The README gains a plain-words section, who sheetpane is for, a roadmap and an FAQ, and says that 0.5.0 is on npm.