Add some docs for why template literals use a chain of .concat() calls.

packages/babel-plugin-transform-es2015-template-literals/src/index.js

@@ -1,4 +1,19 @@
 export default function({ types: t }) {
+  /**
+   * This function groups the objects into multiple calls to `.concat()` in
+   * order to preserve execution order of the primitive conversion, e.g.
+   *
+   *   "".concat(obj.foo, "foo", obj2.foo, "foo2")
+   *
+   * would evaluate both member expressions _first_ then, `concat` will
+   * convert each one to a primitive, whereas
+   *
+   *   "".concat(obj.foo, "foo").concat(obj2.foo, "foo2")
+   *
+   * would evaluate the member, then convert it to a primitive, then evaluate
+   * the second member and convert that one, which reflects the spec behavior
+   * of template literals.
+   */
   function buildConcatCallExressions(items) {
     let avail = true;
     return items.reduce(function(left, right) {