Issue #493: Document that users may need to run composer update after updating blt. (#522)

Author: jrbeeman

readme/updating-blt.md

@@ -8,6 +8,8 @@ If you are already using BLT via Composer, you can update to the latest version
 
 Review and commit changes to your project files. For customized files like `.travis.yml` or `docroot/sites/default/settings.php` it is recommended that you use `git add -p` to select which specific line changes you'd like to stage and commit.
 
+On occasion, updating BLT will result in changes to your project's composer.json file. This can happen when the upstream BLT project adds new dependencies or changes the pinned version of a dependency. In cases like this, BLT will print a message alerting you to run `composer update` an additional time so that you apply the changed dependencies locally. 
+
 Rarely, you may need to refresh your local environment via `blt local:setup` to provision new upstream changes.
 
 ## Updating from a non-Composer-managed (very old) version