Add http-proxy info in install_and_upgrade.md (#4400)

Relates to [#2240](#2240 (comment))

doc/install_and_upgrade.md

@@ -328,6 +328,17 @@ package-indices:
    http: http://mirrors.tuna.tsinghua.edu.cn/hackage/00-index.tar.gz
 ```
 
+## Using an http proxy
+
+To use `stack` behind a http proxy with ip address *IP* and port *PORT*, first set up an environment variable `http_proxy` and then run the stack command. _e.g._
+
+```
+$ export http_proxy=IP:PORT
+$ stack install
+```
+
+Note that on most operating systems, it is not mandatory for programs to follow the "system-wide" http proxy. Some programs, such as browsers, do honor this "system-wide" http proxy setting, while other programs, including bash, do not. That means configuring "http proxy setting" in your Control Panel (Windows) or System Preferences (Mac) would not result in `stack` traffic going through the proxy. 
+
 ## Upgrade
 
 There are essentially four different approaches to upgrade: