Update README.md (#10)

Add file command.

Author: Paulo Henrique

README.md

@@ -86,6 +86,7 @@ Clears content on window.
   <td><a href="#o-lpr">lpr</a></td>
   <td><a href="#p-lpq">lpq</a></td>
   <td><a href="#q-lprm">lprm</a></td>
+  <td><a href="#r-file">file</a></td>
  </tr>
 </table>
 
@@ -225,6 +226,17 @@ Remove something from the printer queue.
 lprm jobnumber
 ```
 
+### r. `file`
+Determine file type. 
+```bash
+file filename
+```
+Example:
+```bash
+$ file index.html
+ index.html: HTML document, ASCII text
+```
+
 ## 1.2. Text Operations
 
 <table>