docs(contrib): Describe how to add a new package

@Muscraft is considering adding a crate and we felt it was best to not
just answer the question of how to do so but to document it!

src/doc/contrib/src/SUMMARY.md

@@ -11,6 +11,7 @@
 - [Design Principles](./design.md)
 - [Implementing a Change](./implementation/index.md)
     - [Architecture](./implementation/architecture.md)
+    - [New packages](./implementation/packages.md)
     - [New subcommands](./implementation/subcommands.md)
     - [Console Output](./implementation/console.md)
     - [Filesystem](./implementation/filesystem.md)

src/doc/contrib/src/implementation/packages.md

@@ -0,0 +1,30 @@
+# New Packages
+
+## Steps
+
+Choose the relevant parent directory
+- `credential/` for credential-process related packages
+- `benches/` for benchmarking of cargo itself
+- `crates/` for everything esle
+
+Run `cargo new <name>`
+- `<name>`:
+  - We tend to use `-` over `_`
+  - For internal APIs, to avoid collisions with third-party subcommands, we can use the `cargo-util-` prefix
+  - For xtasks, we use the `xtask-` prefix
+- `package.rust-version`
+  - Internal packages tend to have a policy of "latest" with a `# MSRV:1` comment to get it auto-updated
+  - Ecosystem packages tend to have a policy of "N-2" with a `# MSRV:3` comment to get it auto-updated
+  - If the right choice is inherited from the workspace, feel free to keep it that way
+- If running without [cargo new automatically adding to workspace](https://github.com/rust-lang/cargo/pull/12779), add it as a workspace member if not already captured by a glob
+
+If its an xtask,
+- Add it to `.cargo/config.toml`s `[alias]` table
+- Mark `package.publish = false`
+
+If published, add the package to `publish.py`, in dependency order
+
+Note: by adding the package to the workspace, you automatically get
+- CI running `cargo test`
+- CI verifying MSRV
+- CI checking for `cargo doc` warnings