增加安装分支的说明

Author: wgbbiao

README.rst

@@ -62,6 +62,11 @@ or Install from github source:
 
     pip install git+git://github.com/sshwsfc/xadmin.git
 
+Install from github source for Django 2.0:
+
+.. code:: bash
+    pip install git+git://github.com/sshwsfc/xadmin.git@django2
+
 Install Requires 
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