Add documentation on updating references in NEWS

CONTRIBUTING.md

@@ -108,6 +108,8 @@ It is encouraged to write all new docstrings in Markdown markup.  If you need to
 Many of the existing docstrings are currently restructured text codeblocks and these will be transitioned to Markdown over time.  RST codeblocks are delineated with the triple-quote (\`\`\`rst  \`\`\`) Makdown codeblock syntax.
 The content of the codeblock is spliced directly into the final restructured text document unmodified.
 
+For new functionality and other substantial changes, add a brief summary to `NEWS.md`. The news item should cross reference the pull request (PR) parenthetically, in the form `([#pr])`; after adding this, run `./julia doc/NEWS-update.jl` from the `julia` directory to update the cross-reference links. To add the PR reference number, first create the PR, then push an additional commit updating `NEWS.md` with the PR reference number.
+
 ### Contributing to core functionality or base libraries
 
 *By contributing code to Julia, you are agreeing to release it under the [MIT License](https://github.com/JuliaLang/julia/tree/master/LICENSE.md).*