[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