Factor out instructions about dependency upgrades into central file

Author: mattwynne

RELEASING.md

@@ -1,13 +1,5 @@
 # Releasing
 
-First, update as many dependencies as you can and test:
-
-    npm run update-dependencies
-    npm test
-
-If the tests fail, update your code to be compatible with the new libraries, or
-revert the library upgrades that break the build.
-
 * Add missing entries to `CHANGELOG.md`
   * Ideally the CHANGELOG should be up-to-date, but sometimes there will be accidental omissions when merging PRs. Missing PRs should be added.
   * Describe the major changes introduced. API changes must be documented. In particular, backward-incompatible changes must be well explained, with examples when possible.