Add more explanation for banker's rounding in AboutNumbers

[vexx32]

Describe "Koan(s) You Suggest"

The banker's rounding example is effective but lacking an explanation as to why that is the default behaviour. It's ultimately PowerShell following .NET's defined defaults there, but deserves an explanation as to why that may have been chosen as the default.

Context "The Scope of the Koans You Suggest"

We'd need to add a few additional comments to this section explaining the rationale behind Banker's rounding and why it exists. It may also be worth showing folx how to round in different ways.

PSKoans/PSKoans/Koans/Introduction/AboutNumbers.Koans.ps1

Lines 61 to 73 in c450201

	Describe "Banker's Rounding" {
	It 'rounds to nearest even number on .5' {
	# Try and guess how PowerShell will round these numbers
	____ | Should -Be ([int]2.5)
	____ | Should -Be ([int]3.34)
	____ | Should -Be ([int]10.61)
	____ | Should -Be ([int]12.7)
	____ | Should -Be ([int]5.5)
	}
	}

Context "Additional Information"

https://twitter.com/jschpp/status/1221810270210809858