Docs on how to write a selection function

[Zsailer]

I realize this is simple, but I don't think we should expect users to dig into the source code to learn how to write a selection function.

Without looking at source code, I wouldn't know what the input, output, arguments, etc would look like to create my own selection function. Further, this is definitely a thing that users will be doing when using this package.

Let's provide a simple documentation page that demonstrates how to write a selection function that the user passes to apply_selection.

I'm imagining some example like this (with details filled in):

def my_selection_function(phenotypes, param_1=None, param_2=None): 
    # Do something to `phenotypes` to get `fitness`  
    return fitness

# Kwargs passed to apply_selection get passed directly to the fitness function.
gpmsm.apply_selection(fitness_function=my_selection_function, param_1=1, param_2=1)