Adding note about simplicity, and link to Robert C. Martin's SOLID principles

Context: https://gitlab.slack.com/archives/C02PF508L/p1548011974586900?thread_ts=1548011974.586900&cid=C02PF508L

Review app: https://adding-solid-to-eng-handbook.about-src.gitlab.com/handbook/engineering/#code-quality-and-standards

CC @winh @lbennett