-DZMK_CONFIG Documentation (#255)

feat(docs): `-DZMK_CONFIG` Documentation

docs/docs/dev-build.md

@@ -84,6 +84,19 @@ 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.
 
+### 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#github-repo) by adding 
+`-DZMK_CONFIG="C:/the/absolute/path/config"` to your `west build` command. **Notice that this path should point to the folder labelled `config` within your `zmk-config` folder.**
+
+
+For instance, building kyria firmware from a user `myUser`'s `zmk-config` folder on Windows 10 may look something like this:
+
+```
+west build -b nice_nano -- -DSHIELD=kyria_left -DZMK_CONFIG="C:/Users/myUser/Documents/Github/zmk-config/config"
+```
+
+
 ## Flashing
 
 Once built, the previously supplied parameters will be remembered so you can run the following to flash your