[chore][CONTRIBUTING.md] Add reference to addlicense command (#33635)

When adding new components, new files are introduced. These files need
to include the license header, which is done by running the `make
addlicense` commnand. This should be documented for users to know what
they need to do when introducing new components and files.

Also, [here's an
example](#33458 (comment))
where it wasn't obvious that this command was necessary.

CONTRIBUTING.md

@@ -225,6 +225,7 @@ When submitting a component to the community, consider breaking it down into sep
     * `make generate`
     * `make multimod-verify`
     * `make generate-gh-issue-templates`
+    * `make addlicense`
 * **Second PR** should include the concrete implementation of the component. If the
   size of this PR is larger than the recommended size consider splitting it in
   multiple PRs.