add README note re: v4 includes/excludes code breaks Jekyll v3/GHPages

README.md

@@ -75,7 +75,7 @@ However, `img` tags need to point to individual icon files. These must be explic
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-> Although most aren't tracked, 2K+ compiled icons can slow down `jekyll build`, as it copies all assets to `_site`. To speed up local development, you can temporarily uncomment the `exclude` and `include` icons listed in `_congif.yml`, matching what's in `.gitignore`. But you don't need to commit this config, as the deploys will only build from the committed icons.
+> Although most aren't tracked, 2K+ compiled icons can slow down `jekyll build`, as it copies all assets to `_site`. To speed up local development, you can temporarily uncomment the `exclude` and `include` icons listed in `_congif.yml`, matching what's in `.gitignore`. But you don't need to commit this config (it breaks Jekyll v3 / GitHub Pages), as deploys will only build from committed icons.
 
 ## Theming USWDS