feat(docs): Add tip for improving build times

docs/docs/development/build-flash.md

@@ -88,6 +88,10 @@ west build -d build/right -b nice_nano -- -DSHIELD=kyria_right
 
 This produces `left` and `right` subfolders under the `build` directory and two separate .uf2 files. For future work on a specific half, use the `-d` parameter again to ensure you are building into the correct location.
 
+:::tip
+Build times can be significantly reduced after the initial build by omitting all build arguments except the build directory, e.g. `west build -d build/left`. The additional options and intermediate build outputs from your initial build are cached and reused for unchanged files.
+:::
+
 ### Building from `zmk-config` Folder
 
 Instead of building .uf2 files using the default keymap and config files, you can build directly from your [`zmk-config` folder](../user-setup.md#github-repo) by adding