[C#] docs: Update README.md | add steps for multi tenant support in dotnet auth sample documentation

[jemjos]

Linked issues

closes: #2198

Details

Add steps for multi tenant support in dotnet auth.TeamsSSO sample documentation

Change details

Adding equivalent steps mentioned in the js sample in c# sample for adding multi tenant support for SSO sample.

screenshots:

[jemjos]

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[singhk97]

Thanks for opening the PR. Were you able to test these changes with the sample?

[jemjos]

@singhk97 - unfortunately i was not able to test it locally due to some technical issues. But got confirmation from customer that it worked. FYI, its just the same implementation mentioned in JS sample for multi-tenant support in C# context for convenience.