Documentation enhancement - Update readme to include instructions for the dev-develop branch of PHPCompatibilty

[jonathanbossenger]

I'm opening this issue as a suggestion for an enhancement to the readme for this project, which I'm also happy to create a PR for, but I first want to check if this will be useful.

As pointed out in #40 it's possible to alias the develop branch of PHPCompatibility to a 9.x version, which is within the allowed range for PHPCompatibilityWP. I'd like to suggest and I am offering to add these instructions to the readme.

I realize this will need to be removed once PHPCompatibility 10.0.0 is released, but I'd also be prepared to help update the readme again once that happens.

[GaryJones]

PR is welcome from my POV! Thank you!

[jrfnl]

To be honest, I don't think that belongs in the readme, not even temporarily. Issue #40, which contains instructions on how to do this, is a pinned issue, so should be easy enough to find.