book: Add dev guide page about Rust dependencies

doc/book/src/SUMMARY.md

@@ -3,6 +3,8 @@
 [zcashd](README.md)
 - [User Documentation](user.md)
   - [Metrics](user/metrics.md)
+- [Developer Documentation](dev.md)
+  - [Rust in `zcashd`](dev/rust.md)
 - [Design](design.md)
   - [Chain state](design/chain-state.md)
   - ["Coins" view](design/coins-view.md)

doc/book/src/dev.md

@@ -0,0 +1,3 @@
+# Developer Documentation
+
+This section contains documentation aimed at contributors to the `zcashd` codebase.

doc/book/src/dev/rust.md

@@ -0,0 +1,56 @@
+# Rust in `zcashd`
+
+`zcashd` is primarily a C++ codebase, but most new code is being written in Rust
+where possible.
+
+## Adding new dependencies in online-Rust mode
+
+The `zcashd` build system pins all dependencies, and in order to faciliate
+deterministic builds, `cargo` is configured to run in offline mode with vendored
+crates. This means that if, for example, you add the `foobar` crate to
+`Cargo.toml`, you will likely see an error similar to this:
+
+```
+$ cargo check
+error: no matching package named `foobar` found
+location searched: registry `https://github.com/rust-lang/crates.io-index`
+required by package `librustzcash v0.2.0 (/path/to/zcash)`
+```
+
+Instead, you first need to build `zcashd` in online-Rust mode:
+```
+CONFIGURE_FLAGS=--enable-online-rust ./zcutil/build.sh
+```
+
+After doing so, you can add a new dependency as follows:
+
+1. Add the new dependency to `Cargo.toml`.
+2. Run `cargo check` to update the `Cargo.lock` file.
+3. Commit `Cargo.toml` and `Cargo.lock`.
+
+## Using a local Rust dependency
+
+During development, you can use a locally checked out version of a dependency
+by applying a [`cargo` patch](https://doc.rust-lang.org/cargo/reference/overriding-dependencies.html#the-patch-section).
+
+For example, to use a local version of the `orchard` crate that includes a new
+API, add the following patch to `Cargo.toml`:
+
+```
+[dependencies]
+# This dependency is listed with a version, meaning it comes from crates.io; the
+# patch goes into a [patch.crates-io] section.
+orchard = "0.0"
+...
+
+[patch.crates-io]
+# Comment out any existing patch, if present.
+# orchard = { git = "https://github.com/zcash/orchard.git", rev = "..." }
+
+# Add this patch:
+orchard = { path = "../relative/path/to/orchard" }
+```
+
+Usually you can apply a patch to use a locally checked out dependency without
+needing to build `zcashd` in online-Rust mode. However, if your local changes
+include a new dependency, you will need to ensure you are in online-Rust mode.