Add install info for CMake/Hunter

[caseymcc]

[Master] Add install info for CMake/Hunter

Description

Adds information on how to install pybind11 when using CMake/Hunter

Suggested changelog entry:

[henryiii]

Looks like the latest version of pybind11 on hunter is 2.8.1. I remember the update process being slightly annoying with CLI11, others keep it up to date nowadays and I'm happy I haven't had to push the update in a while (since before it moved to cpp-pm, in fact). It mostly hurts that it's not automated or easy to automate. I'm not against docs here, though I'd push an update to hunter first (after the next release, which probably will be pretty soon because there are a couple of CMake issues that should be fixed soon (PRs open)). I am worried about this growing forever - maybe we could make this a "details" expander? Maybe after updating the docs formatting (which is on my long-term todo list)?