Add a link to Crockford's Base 32 definition (or copy it)

[mattbishop]

Because Crockford's base 32 definition is central to ULID encoding, the spec should point to the correct source of truth:

https://www.crockford.com/base32.html

Notably, not all implementations alias i, I, o, and O to 1 and 0, respectively.

Also, it would be good to clarify the acceptability of a check symbol.