Add "do not delete" note for legacy assets (#2065)

Files are still required for old versions of the Node.js docs.

Ref: #2041

Author: fhemberger

static/legacy/README.md

@@ -0,0 +1,15 @@
+# DO NOT REMOVE THIS DIRECTORY
+
+Those assets are still being used by legacy versions of the Node.js docs.  
+Everything up to v0.10.28 is a copy of the entire website, e.g.:
+
+- https://nodejs.org/docs/v0.10.26/
+- https://nodejs.org/docs/v0.10.28/ (different design)
+
+Those are the first versions with just the API docs and the changelog only:
+
+- https://nodejs.org/docs/v0.10.27/
+- https://nodejs.org/docs/v0.10.29/
+
+The actual URL rewriting is done in the nodejs.org nginx config:  
+https://raw.githubusercontent.com/nodejs/build/master/setup/www/resources/config/nodejs.org