docs: add proxy usage reference

-refs: yagop#766 
-by @97thjingba

Author: 97thjingba

.gitignore

@@ -7,3 +7,4 @@ output.md
 output/
 lib/
 lib-doc/
+.DS_Store

doc/help.md

@@ -110,6 +110,7 @@ Sources:
 
 * Issue [#122](https://github.com/yagop/node-telegram-bot-api/issues/122)
 * Issue [#253](https://github.com/yagop/node-telegram-bot-api/issues/253)
+* Issue [#766](https://github.com/yagop/node-telegram-bot-api/issues/766)
 
 <a name="new-feature"></a>
 ### Can you add feature X to the library?