Add section on template literal tags (mbeaudru#53)

* Add section on template literal tags

Fix mbeaudru#52.

* Fixed code indents, added sample and ref

Author: Glutnix

readme.md

@@ -1027,6 +1027,48 @@ const name = "Nick";
 - [String interpolation - ES6 Features](http://es6-features.org/#StringInterpolation)
 - [ES6 Template Strings - Addy Osmani](https://developers.google.com/web/updates/2015/01/ES6-Template-Strings)
 
+### Tagged template literals
+
+Template tags are *functions that can be prefixed to a [template literal](#template-literals)*. When a function is called this way, the first parameter is an array of the *strings* that appear between the template's interpolated variables, and the subsequent paremeters are the interpolated values. Use a spread operator `...` to capture all of them. [(Ref: MDN)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals#Tagged_template_literals).
+
+> **Note :** A famous library named [styled-components](https://www.styled-components.com/) heavily relies on this feature.
+
+Below is a toy example on they work.
+```js
+function highlight(strings, ...values) {
+  const interpolation = strings.reduce((prev, next) => {
+    return prev + next + (values.shift() || "");
+  }, "");
+
+  return `<mark>${interpolation}</mark>`;
+}
+
+const condiment = "jam";
+const meal = "toast";
+
+highlight`I like ${condiment} on ${meal}.`;
+// "<mark>I like jam on toast.</mark>"
+```
+
+A more interesting example: 
+```js
+function comma(strings, ...values) {
+  return strings.reduce((prev, next) => {
+    let value = values.shift() || [];
+    value = value.join(", ");
+    return prev + next + value;
+  }, "");
+}
+
+const snacks = ['apples', 'bananas', 'cherries'];
+comma`I like ${snacks} to snack on.`;
+// "I like apples, bananas, cherries to snack on."
+```
+
+#### External resources
+- [Wes Bos on Tagged Template Literals](http://wesbos.com/tagged-template-literals/)
+- [Library of common template tags](https://github.com/declandewet/common-tags)
+
 ### Imports / Exports
 
 ES6 modules are used to access variables or functions in a module explicitly exported by the modules it imports.