Update documentation for adding custom plugins

docs/cn/documentation.md

@@ -255,6 +255,14 @@ SpaceVim 根据需要定义了很多临时快捷键，
     merged = false
 ```
 
+也可以使用仓库克隆的地址，比如：
+
+```toml
+[[custom_plugins]]
+    repo = "https://gitlab.com/code-stats/code-stats-vim.git"
+    merged = false
+```
+
 `one_cmd` 选项使得这个插件延迟加载。
 该插件会在第一次执行 `ColorHighlight` 或者 `ColorToggle`
 命令时被加载。

docs/documentation.md

@@ -279,6 +279,15 @@ to the `custom_plugins` section:
     merged = false
 ```
 
+You can also use the url of the repository, for example:
+
+```toml
+[[custom_plugins]]
+    repo = "https://gitlab.com/code-stats/code-stats-vim.git"
+    merged = false
+```
+
+
 `on_cmd` option means this plugin will be loaded only when the following commands are called.
 
 `merged` option is used for merging plugins directory. When `merged` is `true`, all files in