Add Neovim to editors.

[theherk]

This was originally added in #68, but it was only added to README-en.md, which appears to have gone away, so I'm adding it to the main list.

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Types of Changes

What types of changes does your PR introduce? Put an x in all boxes that apply

• [x] New addition to list
• [ ] Fixing bug in existing item in list
• [ ] Removing item from list
• [ ] Changing structure (organization) of list

Proposed Changes

As described in the commit, this was added but not to the main README.md.

PR Checklist

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• [x] I have read the CONTRIBUTING guide
• [x] I have explained why this PR is important
• [x] I have added necessary documentation (if appropriate)