[ecash-lib-wasm] Add readme with installation instructions

Summary:
I've fielded a few questions now on how to resolve the wasm related dependency errors when trying to run Cashtab after some of the dependency modules have been updated.

This is the only repo without a readme - add one.

Test Plan: Plonk into https://jbt.github.io/markdown-editor/ and proof read

Reviewers: bytesofman, #bitcoin_abc, Fabien

Reviewed By: bytesofman, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17774