Update README.md with PHP 7.3 version constraint (or at least note it).

[joshua-graham-adelphi]

I've just noticed this morning the project recommend installation via composer using govcms/govcms8-project.

This project "govcms/govcms8-project" has minimum PHP requirement of 7.3. (govcms/govcms8 is PHP 7.1)

So user downloading on on PHP 7.1 will get old version (1.0.6 instead of 1.x-dev or later)

I note that "ignore platform requirements" is a workaround.

composer.phar" create-project --stability dev --prefer-dist --ignore-platform-reqs govcms/govcms8-project

Please either update README and/or decrease/version accordingly.

[joshua-graham-adelphi]

Update Schedule transitions v2 now has PHP 7.3 requirement - composer.json needs updating.

[pandaskii]

Agree that we need a PR to PHP version constraint

[joshua-graham-adelphi]

PR #600