inculde an example of the config

includes an example of the config in dev.secret.exs.example.

apps/explorer/config/dev.secret.exs.example

@@ -1 +1,13 @@
 use Mix.Config
+
+# Configure your database
+config :explorer, Explorer.Repo,
+  adapter: Ecto.Adapters.Postgres,
+  database: "explorer_dev",
+  hostname: "localhost",
+  username: "postgres",
+  password: "<REPLACE WITH THE PASSWORD YOU CHOSE>",
+  loggers: [],
+  pool_size: 20,
+  pool_timeout: 60_000,
+  timeout: 80_000