Provide guideline about how to customize the look&feel of Harbor UI

support customizing product name
support customizing logo
support customizing login background image
support customizing about text with different languages

docs/customize_look&feel_guide.md

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+# Customize the look & feel of Harbor
+
+The primary look & feel of Harbor supports to be customized with several simple steps. All the relevant customization in configurations are saved in the `setting.json` file under `$HARBOR_DIR/src/ui_ng/src` folder with `json` format and will be loaded when Harbor is launched. 
+
+## Configure
+Open the `setting.json` file, you'll see the default content as shown below:
+
+```
+{
+  "headerBgColor": "#004a70",
+  "headerLogo": "",
+  "loginBgImg": "",
+  "product": {
+    "title": "VMware Harbor",
+    "company": "vmware",
+    "name": "Harbor",
+    "introductions": {
+      "zh-cn": "",
+      "es-es": "",
+      "en-us": ""
+    }
+  }
+}
+```
+
+Change the values of configuration if you want to override the default style to your own. Here are references:
+
+* **headerBgColor**: Background color of the page header, support either HEX or RGB value. e.g: `#004a70` and `rgb(210,110,235)`.
+* **headerLogo**: Name path of the logo image in the header, e.g: 'logo.png'. The image file should be put in the `images` folder.
+* **loginBgImg**: Name path of the background image displayed in the login page, e.g: 'login_bg.png'. The image file should be put in the `images` folder. Suggest the size of this image should be bigger than 800px*600px.
+* **Product**: Contain metadata / description of the product.
+  - **title**: The full product title displayed in the login page.
+  - **company**: Name of the company publishing the product.
+  - **name**: Name of the product.
+  - **introductions**: The introduction about the product with different languages, which are displayed in the `About` dialog.
+
+## Build
+
+Once the `setting.json` configurations has been updated, re-[build](#configure) your product to apply the new changes.