Add release instructions based on what dfreedm and I figured out. (#5743)

Author: rictic

RELEASING.md

@@ -0,0 +1,16 @@
+## How to release a new version of Polymer
+
+- `npm ci`
+- `npm i -g np`
+- `npx gulp update-version`
+- Update CHANGELOG.md
+- Edit package.json back to the previous version so that `np` can be the one
+  to bump the version.
+- `np --yolo --preview`
+  - Ensures that you have permission, does a final check of all the steps.
+- `npm pack` and diff the tgz's contents against the previous version's.
+  We release a new version of Polymer rarely enough that it's worth it to be
+  careful.
+- `np --yolo`
+- After it completes successfully it will open the GitHub releases page for
+  your new version. Edit the changelist there to remove irrelevent entries and publish.