Merge pull request #3540 from mjauvin/system-proxy

document proxy environment variable for system proxy

qutebrowser/browser/network/proxy.py

@@ -61,6 +61,8 @@ class ProxyFactory(QNetworkProxyFactory):
         """
         proxy = config.val.content.proxy
         if proxy is configtypes.SYSTEM_PROXY:
+            # On Linux, use "export http_proxy=socks5://host:port" to manually set system proxy
+            # ref. http://doc.qt.io/qt-5/qnetworkproxyfactory.html#systemProxyForQuery
             proxies = QNetworkProxyFactory.systemProxyForQuery(query)
         elif isinstance(proxy, pac.PACFetcher):
             proxies = proxy.resolve(query)