Document how PRs and docs should be referenced in release notes

Author: AlexeySachkov

sycl/doc/developer/ContributeToDPCPP.md

@@ -60,7 +60,10 @@ There are no strict guidelines on how to structure release notes, but for
 consistency it is better to follow the existing structure minimal changes. The
 structure we have been using so far is split by change type (i.e. new
 features and bug fixes) and then sub-split by component (i.e. compiler,
-runtime). Please use past tense when describing your change.
+runtime). Please use past tense when describing your change and leave a
+reference to your PR using `intel/llvm#NNNNN` syntax. If you want to reference
+an in-tree document (like an extension spec, for example), it must be
+referenced using a permalink so that it stays actual even if document is moved.
 
 When in doubt, you can always ask reviewers for help/guidance/suggestions.