add sourcemaps note

Author: filipesilva

Diff for: public/docs/ts/latest/cookbook/third-party-lib.jade

@@ -73,6 +73,9 @@ include ../_util-fns
   - `es2015`: a FESM containing ES2015 bundle.
   - `typings`: TypeScript and the AOT compiler will look at this entry for metadata.
 
+  In addition, a minimized version of the UMD bundle should also be provided, as well as full
+  sourcemaps (all the way back to the source) for all JS bundles.
+
   A library should have the following file layout when published:
 
 .filetree