Add note to the readme about the new --retry flag

Author: dbkegley

README.md

@@ -823,6 +823,9 @@ Once the content items have been added, you may initiate a build
 using the `rsconnect content build run` subcommand. This command will attempt to
 build all "tracked" content that has the status `NEEDS_BUILD`.
 
+> To re-run failed builds, use `rsconnect content build run --retry`. This will build
+all tracked content in any of the following states: `[NEEDS_BUILD, ABORTED, ERROR, RUNNING]`.
+
 ```bash
 rsconnect content build run
 # [INFO] 2021-12-14T13:02:45-0500 Initializing ContentBuildStore for https://connect.example.org:3939