Add valid args

Signed-off-by: Adam Warner <[email protected]>

README.md

@@ -213,6 +213,11 @@ Occaisionally you may need to try an alternative branch of one of the components
 
 The preferred method is to clone this repository and rebuild the image with the custom branch name passed in as an arg, e.g `docker buildx build src/. --tag pihole_custom --build-arg CORE_BRANCH=custombranchname --no-cache`, and then redeploy your stack with this new image (In this case you should have a local image named `pihole_custom`, but you can call it whatever you want)
 
+Valid args are:
+ - `CORE_BRANCH`
+ - `WEB_BRANCH`
+ - `FTL_BRANCH`
+
 ### Pi-hole features
 
 Here are some relevant wiki pages from [Pi-hole's documentation](https://github.com/pi-hole/pi-hole/blob/master/README.md#get-help-or-connect-with-us-on-the-web).  The web interface or command line tools can be used to implement changes to pihole.