Add how to use SOCKS proxy (#48)

Add all_proxy env variable to user-manual.md

Author: masoudd

docs/user-manual.md

@@ -319,12 +319,16 @@ Advanced topics
 
 gPodder includes a command-line interface. The command is called `gpo`. You can get a list of possible actions by running the command without any parameters.
 
-### Using a HTTP proxy server
+### Using a HTTP or SOCKS proxy server
 
 You can use a HTTP proxy server for downloading episodes and feeds. Newer versions of gPodder do not provide a way to do so in the GUI, but respect the environment variable **http\_proxy**. How to set the **http\_proxy** environment variable in different operating systems is described here:
 
 -   [Setting http\_proxy in Windows, Mac OS X and Linux/Unix](http://docs.activestate.com/activeperl/5.10/faq/ActivePerl-faq2.html#setting_http_proxy)
 
+You can use a SOCKS proxy server by setting the enronment variable **all\_proxy**. If you want the DNS to be resolved through the proxy, use `socks5h://` instead. See [urllib3 docs](https://urllib3.readthedocs.io/en/stable/advanced-usage.html#socks-proxies) for more details.
+
+    export all_proxy=socks5://hostname:port
+
 If you want to use a proxy for gPodder, but don't want to use a proxy for other applications, you can create a short shell script:
 
     #!/bin/sh