Update fee -> feeSubunits in README.md

[SiennaSaito]

#105

Updates references to fee in the readme to match types.ts.
This is a small portion of the work needed to update the readme before the schema is also changed to match.

[michaelneale]

thanks @SiennaSaito - wonder given that the types are now more rigorously defined if we could make the README automatically updated (or perhaps automated in some other way to not be repeated? if you have any ideas).

[SiennaSaito]

Thanks @michaelneale!

If we're happy with adding the descriptions for each property to types.js, we could come up with a script that prints each type into a table and cats to stdout.

That'd get you 80% of the way there without having to worry about CI and templates etc.
If you wanted to hook it up with CI later then you could have CI update the readme automatically with each merge to main.

I'll spin up an issue so we can talk about it more ☀️

[SiennaSaito]

Issue created - #115