Add lines to COPYING [skip ci]

Author: chrisrichardson

COPYING

@@ -672,3 +672,9 @@ may consider it more useful to permit linking proprietary applications with
 the library.  If this is what you want to do, use the GNU Lesser General
 Public License instead of this License.  But first, please read
 <http://www.gnu.org/philosophy/why-not-lgpl.html>.
+
+
+Note: Individual files contain the following tag instead of the full license text.
+
+SPDX-License-Identifier:    LGPL-3.0-or-later
+This enables machine processing of license information based on the SPDX License Identifiers that are here available: http://spdx.org/licenses/

dolfin/common/ArrayView.h

@@ -2,7 +2,7 @@
 //
 // This file is part of DOLFIN (www.fenicsproject.org)
 //
-// SPDX-License-Identifier: LGPL-3.0-or-later
+// SPDX-License-Identifier:    LGPL-3.0-or-later
 
 #pragma once