Add tips for developers for NPM CLI

[Blackbaud-SteveBrush]

During our UI Guild meeting we talked about improving the Getting Started guide to instruct users to:

• Run skyux upgrade frequently and monitor the output (especially peer dependency warnings).
• Run npm audit frequently to check for any security vulnerabilities.

[blackbaud-johnly]

Addressed in https://github.com/blackbaud/skyux2-docs/pull/1031.