Add encoding algorithms to the specification

[drinckes]

Fixes #718

This updates the specification to include both the reverse and forward encoding algorithms.

Most implementations currently use the reverse algorithm, but the forward one might be slightly more efficient since it only ever needs to compute the requested number of digits. (The existing reverse approach must calculate all 15 digits if 11 or more digits are requested, or at least 10 regardless of what is requested.)

There are some minor changes due to formatting the markdown with the Prettier VSCode extension.

[drinckes]

@bocops , @fulldecent (and anyone else) would appreciate comments before committing.

[fulldecent]

Great to see updates to the spec are in scope.

Please give me a little more time I'd like to get through all this.