docs: reiterate building from `zmk/app/` (#1031)

Co-authored-by: Cem Aksoylar <caksoylar@users.noreply.github.com>

docs/docs/development/build-flash.md

@@ -99,6 +99,10 @@ For instance, building kyria firmware from a user `myUser`'s `zmk-config` folder
 west build -b nice_nano -- -DSHIELD=kyria_left -DZMK_CONFIG="C:/Users/myUser/Documents/Github/zmk-config/config"
 ```
 
+:::caution
+The above command must still be invoked from the `zmk/app` directory as noted above, rather than the config directory. Otherwise, you will encounter errors such as `ERROR: source directory "." does not contain a CMakeLists.txt; is this really what you want to build?`. Alternatively you can add the `-s /path/to/zmk/app` flag to your `west` command.
+:::
+
 In order to make your `zmk-config` folder available when building within the VSCode Remote Container, you need to create a docker volume named `zmk-config`
 by binding it to the full path of your config directory. If you have run the VSCode Remote Container before, it is likely that docker has created this
 volume automatically -- we need to delete the default volume before binding it to the correct path. Follow the following steps: