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
  * Point your browser to http://path/to/mantisbt/admin/check.php to ensure that
    your webserver is compatible with MantisBT and configured correctly
 
+ * 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!
@@ -50,6 +54,10 @@ UPGRADING
  * Point your browser to http://path/to/_NEW_mantisbt/admin/check.php to ensure that
    your webserver is compatible with MantisBT and configured correctly
 
+ * 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.
+
  * To restore your original MantisBT URL operation it is best at this point
    to delete the old MantisBT directory and rename the directory you
    extracted the tarball to to the original directory name.