fix a small typo in README.md
Merge pull request #670 from qqshfox/patch-1

fix a small typo in README.md
allow required param with predefined set of values to be nil,
fixed #635
fix syntax in README.md

The `requires` statement should be in a `params` block according to #560.
Merge pull request #671 from dm1try/fix_required_params_inside_optional_group

allow required param with predefined set of values to be nil inside optional group
Merge pull request #672 from qqshfox/patch-2

fix syntax in README.md