Adding postcss plugin load order explanation

Editor assessment

Documenting plugin load order is a worthwhile addition, and the docs type without a scope suits a general-guides change. The subject's grammar is the notable flaw: 'adding' is a gerund, not the imperative the community expects, so it should read 'add postcss plugin load order explanation'—a small thing, but imperative mood keeps generated changelogs and manual scans consistent. 'Explanation' as the head noun is also slightly awkward; 'document postcss plugin load order' would be tighter and shorter. There is no body, which is acceptable for a self-contained docs edit, though naming which guide or section gained the explanation would aid navigation. No version implications arise from documentation, so tooling is unaffected. Mechanically valid, humanly a little off-key.