Add doc for embedding install location options in apphost

[elinor-fung]

There have been numerous requests around being able to customize how/where the .NET root path will be determined for a an application. This document describes a proposed mechanism for basic configuration of how apphost will search for the .NET install location.

This is intended to be a step towards a broader (not-yet-designed) SDK experience for building an application or set of applications with a customized .NET runtime path. It does not attempt to tackle the problem of the fuller experience.

cc @AaronRobinsonMSFT @richlander @TravisEz13 @dotnet/appmodel

[vitek-karas]

One comment:
(this might be there, but it's not super clear) If the app relative path is turned on, the apphost should only look there, it should not continue searching for global and/or registered runtimes.
Personally, I could see use cases for both where it does not search and also where it does, but the cases I've seen mostly seem to ask for the case where it doesn't search anymore (VS is a good example).
That said, allowing it to continue the search would make the apphost usable even without the runtime in the right place, so F5 and alike would work - but it could also make it look like everything is correct and the app breaks after deployment, which is not great either.

[elinor-fung]

If the app relative path is turned on, the apphost should only look there, it should not continue searching for global and/or registered runtimes.

My intent was that it would only look there by default - but allow the search locations to be explicitly set to something like AppRelative;Global. So from the SDK side, if someone just sets AppRelativeDotNet, it will build an apphost that is set to only search app-relative and from the apphost side it searches based on whatever flags are set.