Policy on supported codebase->analysis_php_version_id version

[SCIF]

Hi there,

I'd like to clarify what is the policy on supported PHP versions of the codebase being analysed. It sounds logically to me to have min supported version of the codebase the same as required to run Psalm itself, but that's not neccessary the case at all times as psalm could be ran using Docker image so I believe it worth to specify supported PHP versoin of codebase somewhere on README.

What do you think? Thanks!