docs: updates --routing-url to optional

CHANGELOG.md

@@ -32,6 +32,19 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
  [pull/487]: https://github.com/apollographql/rover/pull/487
 
 ## 🚀 Features
+
+- **`--routing-url` is now an optional argument to `rover subgraph publish` - [EverlastingBusgtopper], [issue/169] [pull/484]**
+
+ When publishing a subgraph, it is important to include a routing URL for that subgraph, so your graph router
+ knows where to route requests for types in a subgraph. Previously, you had to specify this argument on
+ every `rover subgraph publish`, but now it acts as an upsert, meaning you must include it on your first
+ `rover subgraph publish`, but subsequent publishes will retain the existing routing URL for a subgraph
+ if `--routing-url` is not specified.
+
+ [EverlastingBusgtopper]: https://github.com/EverlastingBusgtopper
+ [pull/484]: https://github.com/apollographql/rover/pull/484
+ [issue/169]: https://github.com/apollographql/rover/issues/169
+
 ## 🐛 Fixes
 ## 🛠 Maintenance
 

docs/source/subgraphs.md

@@ -180,9 +180,12 @@ Alternatively, you can provide `-`, in which case the command uses an SDL string
 
 <td>
 
-**Required.** Used by a gateway running in [managed federation mode](https://www.apollographql.com/docs/federation/managed-federation/overview/).
+**Optional.** Used by a gateway running in [managed federation mode](https://www.apollographql.com/docs/federation/managed-federation/overview/).
 
-If you're running a subgraph that hasn't been deployed yet or isn't using managed federation, you can pass a placeholder URL or leave the flag empty.
+The first time you publish a subgraph, this argument is required (though you can set the value to an empty string if your subgraph has not yet been deployed, or if you aren't using managed federation).
+
+After your first publish, this argument is no longer required, and your graph router will continue to route requests
+for this subgraph to the routing URL it recorded from your first publish.
 
 </td>
 </tr>