Updated PBJ config, added viewability-vendors description

prebid-server/developers/add-new-bidder-java.md

@@ -110,6 +110,7 @@ Modify this template for your bid adapter:
 - Change the `modifying-vast-xml-allowed` value to `false` if you'd like to opt out of video impression tracking. It defaults to `true`.
 - Change the `pbs-enforces-ccpa` to `false` if you'd like to disable ccpa enforcement. Defaults to `true`.
 - Change the `vendor-id` value to id of your bidding server as registered with the [GDPR Global Vendor List (GVL)](https://iabeurope.eu/vendor-list-tcf-v2-0/). Leave this as `0` if you are not registered with IAB Europe.
+- Choose the `supported-vendors` constants: These constants should be unique. The list of existing vendor constants can be found [here](https://github.com/prebid/prebid-server-java/blob/master/src/main/java/org/prebid/server/bidder/ViewabilityVendors.java).
 - Remove the `capabilities` (app/site) and `mediaTypes` (banner/video/audio/native) combinations which your adapter does not support.
 - Change the `cookie-family-name` to the name which will be used for storing your user sync id within the federated cookie. Please keep this the same as your bidder name.
   If you implemented a user syncer, you'll need to provide a default endpoint.