feat: rename default method to randomUUID()

Discussion in #3 yielded the following conclusions as of 2019-11-25:

- We only want to support v4 UUIDs initially
- Exporting them as uuid() might cause confusion should we ever want to
  support other algoritihms in the future.
- Using "v4" in the name seems to cause more confusion than
  clarification since the whole "Version" terminology in the RFC is
  pretty confusing per se.
- Using the term "random" to describe v4 UUIDs yielded generally
  positive feedback and other languages/libraries seem to use it as
  well.
- There's general agreement that we should not support algorithms other
  than v4 random UUIDs for now.

README.md

@@ -42,9 +42,9 @@ developers from security pitfalls.
 
 The UUID standard library provides an API for generating RFC 4122 identifiers.
 
-The default export of the UUID library is the
-[version 4 Algorithm](https://tools.ietf.org/html/rfc4122#section-4.4), and returns the string
-representation _(as described in RFC-4122)_.
+The only export of the UUID library that is initially supported is a method which implements the
+[version 4 "Algorithm for Creating a UUID from Truly Random or Pseudo-Random Numbers"](https://tools.ietf.org/html/rfc4122#section-4.4),
+and returns the string representation _(as described in RFC-4122)_.
 
 ```js
 // We're not yet certain as to how the API will be accessed (whether it's in the global, or a