Update 'README.md'

README.md

@@ -10,7 +10,31 @@ $ cd /etc/apt/sources.list.d
 $ ls 
 docker.list  elementary.list  owncloud.list  patches.list
 ```
+so first we need to list the repositories to determine which will be removed:
+```
+$ sudo apt-key list
+
+/etc/apt/trusted.gpg
+--------------------
+pub   rsa2048 2015-09-28 [SC] [expires: 2022-05-28]
+      06D7 EADE 708A 40FA 136E  B454 0700 205D FD41 A71A
+uid           [ unknown] devel OBS Project <devel@s2.owncloud.com>
+
+/etc/apt/trusted.gpg.d/elementary.key.asc
+-----------------------------------------
+pub   rsa1024 2011-10-01 [SC]
+      6C87 69CE DC20 F5E6 6C3B  7D37 BF36 996C 4E1F 8A59
+uid           [ unknown] Launchpad PPA for elementary OS team
+```
+
+in this case we will delete the owncloud repository `06D7 EADE 708A 40FA 136E  B454 0700 205D FD41 A71A` by using:
+
+```
+$ sudo apt-key del "06D7 EADE 708A 40FA 136E  B454 0700 205D FD41 A71A"
+OK
+
+$ sudo apt-get update 
+```
 
-and you can remove with `rm` commad, in my case I added by error the owncloud repository, thus, I have to remove it: