Document why architecture targets use separate crates

arch/README.md

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+# Architecture crates
+
+Each subdirectory is a small crate that instantiates the generic linker for
+one target architecture. Separate crates let Cargo compile the architectures
+in parallel. Instantiating all targets in one crate leaves a long serial
+compilation step.
+
+The shared linker implementation lives in [`src/`](../src/), with
+architecture-specific code in [`src/arch/`](../src/arch/). These crates provide
+entry points that the [`cli/`](../cli/) executable selects for the input
+files' target.