Minor addition to release docs checklist

docs/release-instructions.md

@@ -7,6 +7,7 @@
 1. (Optional) Update minor dependencies in package.json
    - `npx npm-check-updates`
    - or `npm outdated` + `npm update --save`
+1. Stable release only: make sure there aren’t any `@11ty/*` dependencies on pre-release versions alpha/beta/canary
 1. If the minimum Node version changed, make sure you update `package.json` engines property.
    - Make sure the error message works correctly for Node versions less than 10.
      - 0.12.x+ requires Node 10+