Please add to the Usage in the README

[Christopher Wells]

May I suggest you add the following to the README (because I had to look beyond the README to discover this):

• There are intValue and value properties
• You can construct with negative numbers, and negative strings like "-1" or "(1)"
• You can compare as described here
• Using TypeScript can import using import currency from 'currency.js';
• TypeScript type definitions are already included in the package

[Chew Han Xiang (Developer)]

@scurker Can I do a PR to update the README with the above suggestions? I think they are quite useful.