Modify dev instructions for multi-platform usage

Add extra notes to encourage platform-aware changes.

README.md

@@ -103,9 +103,14 @@ To add a software program to `nextstrain/base`, follow steps in this order:
    `linux` and `amd64`/`x86_64`) is available on the software's website (e.g.
    GitHub release assets). If available, add a download command to the section
    labeled with `Download pre-built programs`.
+    - If a pre-built binary supporting `linux/arm64` (name contains `linux` and
+      `arm64`/`aarch64`) is also available, that should be used conditionally on
+      `ARG`s `TARGETPLATFORM` or `TARGETOS`+`TARGETARCH` in the Dockerfile. See
+      existing usage of those arguments for examples.
 4. The last resort is to build from source. Look for instructions on the
    software's website. Add a build command to the section labeled with `Build
-   programs from source`.
+   programs from source`. Note that this can require platform-specific
+   instructions.
 
 If possible, pin the software to a specific version. Otherwise, add the
 download/install/build command to the section labeled with `Add unpinned