Update README.md

[jordaniza]

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Added additional on_attach and capabilities to nvim-lspconfig

• [ ] Because this PR includes a bug fix, relevant tests have been included.
• [ ] Because this PR includes a new feature, the change was previously discussed on an Issue or with someone from the team.
• [x] I didn't do anything of this.

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The Language server for hardhat works great with the config in the readme, but I found a couple of features missing:

• Error diagnostics were being shown in the editor, but the keybinding to open the modal was not expanding the full error.
• Docstrings and signatures were not being loaded with <K>

Both of these are a bit of a pain to work without, so added the recommended additional lines to the LSP config for neovim.

Let me know what you think.