Improve description of configuration in README.md.

This is for #536, but we may want to keep the issue open for discussion a while longer.

Move the sentence about what the configuration file is for, to the first paragraph.

Make the example `.rubocop.yml` file more realistic by setting parameter values that are different from the default ones.

Clarify the note about `AllCops`/`Excludes`.

Make first paragraph under "Including/Excluding files" more clear and simple.

Make "Inheritance" a subchapter, and try to use the word "inherit" rather than "include" when describing `inherit_from`.

Replace the word folder with directory so we use the same nomenclature throughout the README.

Make "Disabling Cops within Source Code" its own chapter, not under "Configuration", since it has nothing to do with `.rubocop.yml`.