Update subgraphs page with chain slugs

pages/building-with-pgn/subgraphs.mdx

@@ -11,3 +11,5 @@ PGN subgraphs can be deployed on Goldsky; for more information on getting starte
 PGN subgraphs on Goldsky can be deployed in 2 ways:
 * Via CLI from a local subgraph configuration file. If you are familiar with developing subgraphs already, you'll be familiar with this approach; after defining a subgraph lcoally (with a subgraph.yaml file, a schema.graphql file, and the necessary mappings to translate raw event data into the entities defined in the schema), you can deploy subgraphs to Goldsky (once the Goldsky CLI is installed) using `goldsky subgraph deploy <name>/<version> --path`.
 * Via [instant subgraphs](https://docs.goldsky.com/indexing/instant-subgraphs), where you can pass through a contract address and the ABI for that contract. This is a quick-start option that automatically generates the underlying subgraph configuration files on your behalf, making it easy to extract blockchain event data and serve it as an API endpoint without complex setup. Use the `--from-ipfs-hash` flag in the command above instead of `--path`.
+
+Both PGN mainnet and testnet are available at the chain slugs `publicgoods` and `publigoods-testnet` respectively.