Documentation assumes I have yarn installed

[boek]

To make things easier for people coming to RN from rust we may want to call out installing yarn in the getting started guide.

[Johennes]

It is mentioned in the pre-install guide actually: https://github.com/jhugman/uniffi-bindgen-react-native/blob/main/docs/src/guides/pre-installation.md?plain=1#L11

Maybe it needs to be repeated in the getting starting guide?

[jhugman]

This is the second time the "Rust to RN" docs have caught people out. Perhaps we need to start from the very basics, with an Install Node and Yarn section.

[boek]

🤦 not sure how I missed that.

I pulled that section out into an info which might help people like me not gloss over it.

[jhugman]

Fixed by #142