docs(UserGuide): add permissions command to user guide

Closes meganz#719

UserGuide.md

@@ -675,6 +675,27 @@ Modifies user password
 
 Usage: `passwd [oldpassword newpassword]`
 
+### permissions
+Shows/Establish default permissions for files and folders created by MEGAcmd.
+
+Usage: `permissions [(--files|--folders) [-s XXX]]`                             
+
+<pre>
+Permissions are unix-like permissions, with 3 numbers: one for owner, one for group and one for others
+
+Options:
+ --files	To show/set files default permissions.
+ --folders	To show/set folders default permissions.
+ --s XXX	To set new permissions for newly created files/folder. 
+        	 Notice that for files minimum permissions is 600,
+        	 for folders minimum permissions is 700.
+        	 Further restrictions to owner are not allowed (to avoid missfunctioning).
+        	 Notice that permissions of already existing files/folders will not change.
+        	 Notice that permissions of already existing files/folders will not change.
+
+Notice: this permissions will be saved for the next time you execute MEGAcmd server. They will be removed if you logout.
+</pre>
+
 ### preview
 To download/upload the preview of a file.