DOPS-243: Add manifest.txt (#15959)

* DOPS-243: Add manifest.txt

* Update build/release.mk

* Change path

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

build/release.mk

@@ -108,6 +108,9 @@ else
 endif
 	cp NOTICE.txt $(DIST_PATH)
 	cp README.md $(DIST_PATH)
+	if [ -f ../manifest.txt ]; then \
+		cp ../manifest.txt $(DIST_PATH); \
+	fi
 
 	@# Import Mattermost plugin public key
 	gpg --import build/plugin-production-public-key.gpg