Add comment explaining plural handling for defaultExportNames

@lencioni pointed out that this was a little confusing, and I agree.
Adding a comment to explain the use-case a little should help others
reading this code.

lib/ModuleFinder.js

@@ -78,6 +78,11 @@ function defaultExportNames(pathToFile) {
   ];
 
   if (/e?s$/.test(dirName)) {
+    // When the directory has an "s" (or "es") at the end, we assume it's a
+    // plural form of something. We then add a third name to the list. As an
+    // example, if we have './mocks/ModuleFinder.js' as the pathToFile, we add
+    // 'mock-ModuleFinder' here, which will allow the user to find the module
+    // with `MockModuleFinder` as the variable name.
     result.push(`${dirName.replace(/e?s$/, '')}-${fileName}`);
   }
   return result;