update README for Debian installation (#698)

README.md

@@ -32,6 +32,8 @@ refer to the [Wiki page](https://github.com/shadowsocks/shadowsocks/wiki/Feature
 
 - [Debian & Ubuntu](#debian--ubuntu)
     + [Install from repository](#install-from-repository)
+      - [Official repository](#official-repository)
+      - [Unofficial repository](#unofficial-repository)
     + [Build deb package from source](#build-deb-package-from-source)
     + [Configure and start the service](#configure-and-start-the-service)
 - [Fedora & RHEL](#fedora--rhel)
@@ -89,7 +91,21 @@ in the system during compilation and linking.
 
 #### Install from repository
 
-**Note: The repository doesn't always contain the latest version. Please build from source if you want the latest version (see below)**
+**Note: The repositories doesn't always contain the latest version. Please build from source if you want the latest version (see below)**
+
+##### Official repository
+
+Using official repository for Debian unstable:
+
+```bash
+sudo apt update
+sudo apt install shadowsocks-libev
+```
+
+**NOTE**: You may need to uninstall any unofficial debian packages for `shadowsocks-libev`
+to install this one from Debian official repository if you encounter any problem.
+
+##### Unofficial repository
 
 Add GPG public key: