Add messaging around first-run behaviour (#17)

* wip: add messaging

* chore: typo

src/index.js

@@ -33,6 +33,12 @@ if (!existsSync(MANIFEST_FILENAME)) {
     console.log(
         `Package size (${currentStats.packageSize}) adopted as new limit.`,
     )
+
+    if (!isUpdatingManifest) {
+        console.log(
+            '❗ It looks like you ran PackWatch without a manifest. To prevent accidental passes in CI or hooks, packwatch will terminate with an error. If you are running packwatch for the first time in your project, this is expected!',
+        )
+    }
     // If the update flag wasn't specified, exit with a non-zero code so we
     // don't "accidentally" pass CI builds if the manifest didn't exist
     process.exit(isUpdatingManifest ? 0 : 1)