Explain relationship of remaining documents to papers repo.

Linkrot avoidance, in other words.

papers/README.md

@@ -1,3 +1,6 @@
 # papers
 
 These papers have moved to: https://github.com/ipfs/papers
+
+Older versions of the papers are still hosted here to avoid breaking links,
+but please update links and refer to the https://github.com/ipfs/papers repo for the most up-to-date versions.