May be add a section of document for proxy setting.

[lvscar]

How to set proxy

Found your request back-end

There are two options for the network back-end of the
project: one is the 'curl' Unix command, and the other is the Emacs
built-in 'url-retrieve' function. Therefore, there are two ways to set
the proxy.

Please use Ctl-h v to check your current request-backend .

If the value is 'curl', you should set the proxy like this::

(setq request-curl-options '("-x http://YOUR-PROXY-SERVER")) 

if the value is 'url-retrieve', you should set the proxy like:

(setq url-proxy-services
   '(("no_proxy" . "^\\(localhost\\|192\\.168\\..*\\)")
     ("http" . "YOUR-PROXY-SERVER")
     ("https" . "YOUR-PROXY-SERVER")))