Add readme steps on how to validate external packages within a full source-build (#1448)

* Add readme steps on how to validate external packages within a full source-build

* Apply suggestion from @mthalman

Co-authored-by: Matt Thalman <[email protected]>

* Apply suggestion from @mthalman

Co-authored-by: Matt Thalman <[email protected]>

---------

Co-authored-by: Matt Thalman <[email protected]>

Author: MichaelSimons

README.md

@@ -66,6 +66,12 @@ to .NET. The following sections describe how to add/upgrade the various types of
 
 1. If the original binaries have strong name signatures, validate the source built ones have them as well.
 
+1. Open a PR.
+
+1. Trigger a full source build within the VMR from your PR by adding a `/azp run source-build-reference-packages-unified-build` comment.
+   This will validate the new external will build without adding prebuilts.
+   It will also ensure the external does not contain prohibited checked-in binaries.
+
 #### Updating an External Component to a Newer Version
 
 1. Update the `./src/externalPackages/src/<external_repo_dir>` to the desired sha
@@ -94,6 +100,12 @@ to .NET. The following sections describe how to add/upgrade the various types of
 1. Validate the version of the NuGet packages and binaries produced by the build.
    See the contents of `./artifacts/packages/<build_configuration>/Shipping`.
 
+1. Open a PR.
+
+1. Trigger a full source build within the VMR from your PR by adding a `/azp run source-build-reference-packages-unified-build` comment.
+   This will validate the new version will build without adding prebuilts.
+   It will also ensure the new version does not contain prohibited checked-in binaries.
+
 1. After the PR is merged to update a component, coordination is often needed in the darc dependency flows.
    The source-build-reference-packages source may need to flow in at the same time as the cooresponding changes in product repos which take a dependency on the new component version.
    Sometimes it can be easier to add the new upgraded version along side the older version and then delete the old version after all product repos have been upgraded to the new version.