Document how to build the code

DEVELOPMENT.md

@@ -0,0 +1,109 @@
+# Prerequisites
+
+- Rust 1.77 (or newer)
+- NodeJS 20 (or newer) with pnpm 9 (or newer)
+- Ruby 3.3 (or newer)
+
+You should be able to install these however you want. If you are
+looking for suggestions applicable to macOS...
+
+- Rust via [rustup][]
+- NodeJS via [Homebrew][]
+- pnpm via [Homebrew][]
+- Ruby via [RVM][]
+
+[rustup]: https://rustup.rs
+[Homebrew]: https://brew.sh
+[RVM]: https://rvm.io
+
+# Building
+
+CSS and JS assets are bundled with the release binary in order to
+create the crate list HTML page. These assets are not normally checked
+into the repository and need to be built by the developer:
+
+```
+% cargo xtask assets --watch
+```
+
+Once this is running in one window, a normal build via Cargo will suffice:
+
+```
+% cargo build
+```
+
+# Tests
+
+## Registry conformance
+
+This ensures that the files created by Margo function as a valid Cargo
+registry. It drives Margo and Cargo as command line tools, much
+like a user would.
+
+These tests are written in Rust and live in the `conformance` directory. To run them:
+
+```
+% cd conformance
+% cargo run
+```
+
+## UI
+
+This ensures that the HTML output generated by Margo is usable and
+useful. It drives Margo and Cargo as command line tools, much like a
+user would.
+
+These tests are written in Ruby and live in the `integration-tests` directory.
+
+```
+% cd integration-tests
+% bundle exec rspec
+```
+
+# Linting / Formatting
+
+A number of tools are checked in CI.
+
+## Rust
+
+- [Rustfmt][]
+- [Clippy][]
+
+```
+% cargo fmt
+% cargo clippy
+```
+
+[Rustfmt]: https://github.com/rust-lang/rustfmt
+[Clippy]: https://doc.rust-lang.org/clippy/
+
+## TS / CSS / Markdown / YAML
+
+- [TypeScript][]
+
+```
+% pnpm check
+```
+
+[TypeScript]: https://www.typescriptlang.org
+
+## Ruby
+
+- [RuboCop][]
+
+```
+% cd integration-tests
+% bundle exec rubocop
+```
+
+[RuboCop]: https://github.com/rubocop/rubocop
+
+## CSS / Markdown / YAML
+
+- [Prettier][]
+
+```
+% pnpm fmt
+```
+
+[prettier]: https://prettier.io