docs: move CODE_OF_CONDUCT.md to correct location (testing-library#175)

**What**:

Move `CODE_OF_CONDUCT.md` from `other` to root of repo

**Why**:

`README.md` links to this file and expects to find it in the root.

**Checklist**:

- [x] Documentation
- [x] Tests (N/A)
- [x] Ready to be merged <!-- In your opinion, is this ready to be merged as soon as it's reviewed? -->
- [x] Added myself to contributors table <!-- this is optional, see the contributing guidelines for instructions -->

Author: RoystonS