Fix #12607: Update installation instructions regarding admin directory

The /admin/ directory should be removed after installation or upgrading
of MantisBT. The installation instructions did not state this
requirement and therefore it was quite easy for users to leave this
potentially dangerous directory in place on live installations of
MantisBT connected to the Internet.

doc/INSTALL

@@ -31,6 +31,10 @@ INSTALLATION
  * Installation is complete -- you may need to copy the default configuration
    to mantisbt/config_inc.php if your web server does not have write access
 
+ * Remove the admin/ directory from within the MantisBT installation path. The
+   scripts within this directory should not be accessible on a live MantisBT
+   site or on any installation that is accessible via the Internet.
+
 UPGRADING
 
  * Backup your existing installation and database -- really!
@@ -50,5 +54,8 @@ UPGRADING
 
  * Click install/upgrade
 
- * Upgrading is complete
+ * Remove the admin/ directory from within the MantisBT installation path. The
+   scripts within this directory should not be accessible on a live MantisBT
+   site or on any installation that is accessible via the Internet.
 
+ * Upgrading is complete