Add documentation for PathValue and SetPathValue support in README

[catatsuy]

Thank you for maintaining chi!

The support for PathValue and SetPathValue was added in #901, which is a great feature. However, this is not mentioned in the README.

Some users might assume that chi does not support this feature and choose another router instead. Since this is part of Go's standard mechanism, I believe it should be recommended in the documentation.

Would it be possible to update the README to mention this support?

Thank you!