Documentation for IVaultClient registration in DI

[AleksandreJavakhishvili]

Describe the feature request or question
With the current documentation It is not clear how we should register IVaultClient in DI with scoped/transient or singleton scope, I mistakenly added as a singleton, which ended up to have one instance of VaultClient and as It doesn't support token refresh we got {"errors":["permission denied"]} after 24h, It would be better If we add DI section in README.md.

[rajanadar]

done @AleksandreJavakhishvili

[rajanadar]

@AleksandreJavakhishvili please check https://www.nuget.org/packages/VaultSharp/1.7.0.4