Add explicit version instructions to README

[eouw0o83hf]

Background

I am working on implementing ent in a codebase. The testing page of ent specifically recommends using this repository.

go getting this repository installs an invalid version which only fails in a way that's really difficult to analyze. Figuring out what the root cause was and figuring out how to install the correct version has eaten up an unfortunate chunk of my day.

Change

It looks like there's some other work going on in PRs to remediate the version issue. This doesn't pretend to fix that, but gives developers who wind up on this page a really explicit and easy way to solve that versioning problem before it wrecks their days.

Cheers!