add PowerShell to proxies docs.

doc/src/network-proxies.md

@@ -3,18 +3,23 @@
 Enterprise networks often don't have direct outside HTTP access, but enforce
 the use of proxies. If you're on such a network, you can request that `rustup`
 uses a proxy by setting its URL in the environment. In most cases, setting
-`https_proxy` should be sufficient. On a Unix-like system with a shell like
-__bash__ or __zsh__, you could use:
-
-```bash
-export https_proxy=socks5://proxy.example.com:1080 # or http://proxy.example.com:8080
-```
-
-On Windows, the command would be:
-
-```cmd
-set https_proxy=socks5://proxy.example.com:1080
-```
+`https_proxy` should be sufficient. Commands may differ between different
+systems and shells:
+
+ - On a Unix-like system with a shell like __bash__ or __zsh__:  
+   ```bash
+   export https_proxy=socks5://proxy.example.com:1080
+   ```
+ - On Windows [__Command Prompt (cmd)__][cmd]:  
+   ```cmd
+   set https_proxy=socks5://proxy.example.com:1080
+   ```
+ - On Windows [__PowerShell__][ps] (or __PowerShell Core__):  
+   ```cmd
+   $env:https_proxy="socks5://proxy.example.com:1080"
+   ```
+ - Replace `socks5://proxy.example.com:1080` with 
+  `http://proxy.example.com:8080` when an HTTP proxy is used instead.
 
 If you need a more complex setup, `rustup` supports the convention used by the
 __curl__ program, documented in the ENVIRONMENT section of [its manual
@@ -34,3 +39,5 @@ getting an SSL `unknown protocol` error from `rustup` via `libcurl` but the
 command-line `curl` command works fine, this may be the problem.
 
 [curlman]: https://curl.haxx.se/docs/manpage.html
+[cmd]: https://en.wikipedia.org/wiki/Cmd.exe
+[ps]: https://en.wikipedia.org/wiki/PowerShell