Add hint to install typescript package on @iota/client installation

[cbries]

Description of change

A hint how to install TypeScript package which was not found on my development system by npm.

Links to any relevant issues

none

Type of change

• Documentation Enhancement

How the change has been tested

Hint is directly used on the local development system.

Change checklist

Tick the boxes that are relevant to your changes, and delete any items that are not.

• [x] I have followed the contribution guidelines for this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have tested the wiki locally and tested that all external links work