Document how to implement custom reader split functions

[atonks2]

ICL Readers can be created with ReaderOptions, which makes it possible to implement custom bufio.SplitFuncs to handle vendor-specific ICL specifications/variations. These split functions aren't very intuitive to implement, so it would be helpful if ICL had a documentation section explaining how they work, and how to write them for use with its Reader.

Helpful links:

• ICL Reader
• Bufio SplitFunc
• Example ReaderOption implementation

Note: The Reader's scanner is an unexported field, so a SetSplitFunc(split bufio.SplitFunc) method needs to be implemented so consumers of this library can implement their own SplitFuncs using ICL ReaderOptions.