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 subchapter, not under "Configuration", since it has nothing to do with .rubocop.yml.
Merge pull request #540 from jonas054/describe_excludes

Improve description of configuration in README.md.