Make minimal PHP requirement explicit

Problem/Motivation

I composer-required the module in a PHP 7.0-based site, but was greeted with a great big fatal error. Only after that I saw that at the top of the project page there is a quote stating the module requires PHP 7.1. Telling me to RTFM is completely legitimate, although I would like to propose some changes anyway:

Proposed resolution

1. Make the PHP minimal requirement explicit. I think if it's added to the info.yml, it will be automatically picked up in the generated composer.json too, although I am not 100% sure.
2. Change the quote on the project page to a bold-face paragraph instead (although I think this is less relevant if the first point is adressed).

Remaining tasks

1. Agree this is a good idea
2. Create a patch
3. Review
4. Commit

User interface changes

None.

API changes

None.

Data model changes

None.