chore(dev): Add note about generating licenses to CONTRIBUTING.md

Also have the check command output how to fix it to guide contributors.

Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>

docs/CONTRIBUTING.md

@@ -25,6 +25,7 @@ Vector team member will find this document useful.
     - [Flakey tests](#flakey-tests)
       - [Test harness](#test-harness)
   - [Deprecations](#deprecations)
+  - [Dependencies](#dependencies)
 - [Next steps](#next-steps)
 - [Legal](#legal)
   - [Contributor License Agreement](#contributor-license-agreement)
@@ -184,6 +185,13 @@ any pull request with:
 
 When deprecating functionality in Vector, see [DEPRECATION.md](DEPRECATION.md).
 
+### Dependencies
+
+When adding, modifying, or removing a dependency in Vector you may find that you need to update the
+inventory of third-party licenses maintained in `LICENSE-3rdparty.csv`. This file is generated using
+[rust-license-tool](https://github.com/DataDog/rust-license-tool.git) and can be updated using
+`cargo vdev build licenses`.
+
 ## Next steps
 
 As discussed in the [`README`](README.md), you should continue to the following

vdev/src/commands/check/licenses.rs

@@ -9,6 +9,9 @@ pub struct Cli {}
 
 impl Cli {
     pub fn exec(self) -> Result<()> {
-        app::exec("rust-license-tool", ["check"], true)
+        app::exec("rust-license-tool", ["check"], true).map_err(|err| {
+            eprintln!("Run `cargo vdev build licenses` to regenerate the file");
+            err
+        })
     }
 }