Add reference to `tidyfinance` package to TFWR

[christophscheuch]

Add it to preface or getting started?

[patrick-weiss]

I suggest a separate section in "getting started" -> "R package: TidyFinance".

Should we also include some references in the relevant sections (e.g., this download can also be achieved by using "function()")?

[christophscheuch]

So the proposal is:

1. Add subsection to Getting Started about tidyfinance package and how to install
2. Add in-line mentions of tidyfinance functions

What do you think @voigtstefan and @dsforecast?

[voigtstefan]

Sounds good.