docs: describe parameter types & instruct TS usage

[scscgit]

When I was reading current README in a hurry, I felt like the current instructions are ambiguous, e.g. as if the '42' parameter is some length and 'hex' is a value to be converted. The list of supported algorithms also has to be deduced with some assumptions. My suggestion is to solve all these issues together by explicitly documenting the type of this library interface for any newcomers, possibly along with a reminder that the @types/sha.js library is also available (it feels like the "on pure JavaScript" description implies otherwise).

The algorithm parameter is however typed as string right now; I've prepared a PR to address this in https://github.com/DefinitelyTyped/DefinitelyTyped/pull/59757