make markdownlint happier

VENDORED_CODE.md

@@ -1,15 +1,17 @@
+# Use of vendored code in Weave Net
+
 Weave Net is licensed under the [Apache 2.0 license](LICENSE).
 
 Some vendored code is under different licenses though, all of them ship the
 entire license text they are under.
 
- - https://github.com/weaveworks/go-checkpoint
-   can be found in the ./vendor/ directory, is under MPL-2.0.
+- https://github.com/weaveworks/go-checkpoint
+  can be found in the ./vendor/ directory, is under MPL-2.0.
 
- - Pulled in by dependencies are
-   https://github.com/hashicorp/golang-lru (MPL-2.0)
-   https://github.com/hashicorp/go-cleanhttp (MPL-2.0)
-   https://github.com/opencontainers/go-digest (docs are under CC by-sa 4.0)
+- Pulled in by dependencies are  
+  https://github.com/hashicorp/golang-lru (MPL-2.0)  
+  https://github.com/hashicorp/go-cleanhttp (MPL-2.0)  
+  https://github.com/opencontainers/go-digest (docs are under CC by-sa 4.0)
 
 [One file used in tests](COPYING.LGPL-3) is under LGPL-3, that's why we ship
 the license text in this repository.