Add SPDX license reference

Signed-off-by: Tomasz Gromadzki <tomasz.gromadzki@intel.com>

CODING_STYLE.md

@@ -48,9 +48,13 @@ Otherwise, just use `unsigned`, `long`, etc.
 - Make sure you have the right to submit your contribution under the BSD license,
 especially if it is based upon previous work.
 See [CONTRIBUTING.md](https://github.com/pmem/pmdk/blob/master/CONTRIBUTING.md) for details.
-- A copy of the [BSD-style License](https://github.com/pmem/pmdk/blob/master/LICENSE.txt)
-must be placed at the beginning of each source file, script or man page
-(Obviously, it does not apply to README's, Visual Studio projects and \*.match files.)
+- A copy of the SPDX BSD-3-Clause license reference must be placed at the beginning of each
+source file, script or man page. (Obviously, it does not apply to README's, Visual Studio projects
+and \*.match files.)
+```
+/* SPDX-License-Identifier: BSD-3-Clause */
+/* Copyright <year>, <Name/Company name> */
+```
 - When adding a new file to the repo, or when making a contribution to an existing
 file, feel free to put your copyright string on top of it.