Update README.md

[Tiryoh]

What does this implement/fix?

パルスカウンタはI2Cの通信速度を下げなくても問題がない場合があるのでREADMEを変更します。

Does this close any currently open issues?

しません

How has this been tested?

Any other comments?

Checklists

• [x] I have read the CONTRIBUTING guidelines.
• [x] I have checked to ensure there aren't other open Pull Requests for the same change.

[Tiryoh]

related to #13