Add a comment to explain performance optimization

src/processTailwindFeatures.js

@@ -23,6 +23,15 @@ export default function(getConfig) {
       substituteResponsiveAtRules(config),
       substituteScreenAtRules(config),
       substituteClassApplyAtRules(config, processedPlugins.utilities),
+
+      // This quick plugin is necessary to avoid a serious performance
+      // hit due to nodes created by postcss-js having an empty `raws`
+      // value, and PostCSS not providing a default `raws.semicolon`
+      // value. This turns determining what value to use there into an
+      // O(n) operation instead of an O(1) operation.
+      //
+      // The latest version of PostCSS 7.x has this patched internally,
+      // but patching from userland until we upgrade from v6 to v7.
       function(root) {
         root.rawCache = {
           colon: ': ',