improve README.md about licensing * Use meanwhile established `SPDX-FileCopyrightText:` line to give the holder of the usage rights, improve the explanation. * Use `SPDX-License-Identifier:` also here.

#-*-makefile-*-
# Authors: Sascha Wilde <wilde@intevation.de>

.PHONY: all dependencies clean

all:	dependencies
	yarn build

dependencies:
	yarn install --check-files

clean:
	rm -rf ../web