Document converters, mergers, and how they're resolved

[JoshuaKGoldberg]

Overview

For folks interested in contributing to tslint-to-eslint-config, having explanations of the main parts of the code base are essential. docs/Development.md should explain those very core principles.

In short:

• A "converter" is a function that converts a particular TSLint rule's arguments to a description of the equivalent ESLint rule(s) and arguments(s)
• A "merger" is used to figure out how to combine two groups arguments for a particular ESLint rule, which is used when multiple converters output the same ESLint rule