README for sample app

[pauldambra]

it's a while since i ran asp .net so there may be a better way of telling folk to do this

but i wanted to run against local so needed to set these two settings

i'll assume some other people are as silly as me and a note in the README might help 😊

[haacked]

There's instructions for this in the PostHog.AspNetCore README: https://github.com/PostHog/posthog-dotnet/blob/main/src/PostHog.AspNetCore/README.md

However, as I think about it, if users update appsettings.Development.json, they'll have to be careful not to commit those to this repository. So I think we should suggest the commands to run to set the env variables like I do with the ProjectApiKey.

[haacked]

For users of this library, they can put settings in appSettings.json for their own project, but we probably don't want them to do that for our project.

[pauldambra]

cool cool... happy for you to edit this PR or just throw it away and do something else :)

[haacked]

I did something else. Thanks though!