Broken link to Contribution Guidelines in README.md

[noborus]

Issue description

Broken link to Contribution Guidelines in README.md.

-See the [Contribution Guidelines](https://github.com/go-sql-driver/mysql/blob/master/CONTRIBUTING.md) for details.
+See the [Contribution Guidelines](https://github.com/go-sql-driver/mysql/blob/master/.github/CONTRIBUTING.md) for deta
ils.

[shogo82148]

pull request is welcome

[noborus]

You're right.
PR sent.