This is according to our formal, published, policy to only support
EOL PHP after 6 months.
See https://phpspreadsheet.readthedocs.io/en/latest/#php-version-support
Also consolidate documentation in a single place, not in the README,
because it's a pain to maintain almost-duplicated-but-not-really
documentation.
Adrien Crivelli
8968062e8d