FAQ: add third-party iconfont usage (#1265)

FAQ.md

@@ -34,6 +34,27 @@
 请阅读 Vue 文档 [Render Function](http://vuejs.org/v2/guide/render-function.html) 的相关内容。注意，使用 JSX 来写 Render Function 的话，需要安装 `babel-plugin-transform-vue-jsx`，并参照其[文档](https://github.com/vuejs/babel-plugin-transform-vue-jsx)进行配置。
 </details>
 
+<details>
+<summary>如何使用第三方图标库？</summary>
+
+只要修改第三方图标库的前缀（具体方法参阅第三方库的文档），并编写相应的 CSS，即可在 Element 中像使用内置图标一样使用第三方图标。例如，将第三方库的前缀改为 `el-icon-my`，然后在其 CSS 文件中添加：
+```css
+[class^="el-icon-my"], [class*=" el-icon-my"] {
+  font-family:"your-font-family" !important;
+  
+  /* 以下内容参照第三方图标库本身的规则 */
+  font-size: inherit;
+  font-style:normal;
+  -webkit-font-smoothing: antialiased;
+  -moz-osx-font-smoothing: grayscale;
+}
+```
+具体使用时，和 Element 内置的图标用法一样。比如在 `el-input` 中：
+```html
+<el-input icon="my-xxx" />
+```
+</details>
+
 <details>
 <summary>你们的文档怎么偷偷更新了？</summary>
 
@@ -98,6 +119,27 @@ The parameter `row` is the data object of corresponding row.
 Please refer to [Render Function](http://vuejs.org/v2/guide/render-function.html) in Vue's documentation. In addition, if you are writing render functions with JSX, `babel-plugin-transform-vue-jsx` is required. See [here](https://github.com/vuejs/babel-plugin-transform-vue-jsx) for its configurations.
 </details>
 
+<details>
+<summary>How do I use third-party icon font library with Element?</summary>
+
+You just need to modify the class name prefix of the third-party library (see their docs for how to do it), and write some CSS, then you can use them just like you use Element built-in icons. For example, change the prefix to `el-icon-my`, and then add the following to its CSS:
+```css
+[class^="el-icon-my"], [class*=" el-icon-my"] {
+  font-family:"your-font-family" !important;
+  
+  /* The following is based on original CSS rules of third-party library */
+  font-size: inherit;
+  font-style:normal;
+  -webkit-font-smoothing: antialiased;
+  -moz-osx-font-smoothing: grayscale;
+}
+```
+Now you can use them as you do with built-in icons. For example, in `el-input`:
+```html
+<el-input icon="my-xxx" />
+```
+</details>
+
 <details>
 <summary>When do you update documentations of Element?</summary>