message: Document `vim.useSystemClipboard` in README
diff: @@ -118,6 +118,7 @@ These settings are specific to VSCodeVim.
  | vim.disableExtension | Disable VSCodeVim extension. This setting can also be toggled using `toggleVim` command in the Command Palette | Boolean | false |
  | vim.handleKeys | Delegate configured keys to be handled by VS Code instead of by the VSCodeVim extension. Any key in `keybindings` section of the [package.json](https://github.com/VSCodeVim/Vim/blob/master/package.json) that has a `vim.use<C-...>` in the `when` argument can be delegated back to VS Code by setting `"<C-...>": false`. Example: to use `ctrl+f` for find (native VS Code behaviour): `"vim.handleKeys": { "<C-f>": false }`. | String | `"<C-d>": true` |
  | vim.overrideCopy | Override VS Code's copy command with our own, which works correctly with VSCodeVim. If cmd-c/ctrl-c is giving you issues, set this to false and complain [here](https://github.com/Microsoft/vscode/issues/217). | Boolean | false |
  +| vim.useSystemClipboard | Use the system clipboard register (`*`) as the default register | Boolean | false |
  | vim.searchHighlightColor | Set the color of search highlights | String | `findMatchHighlightBackground` ThemeColor |
  | vim.startInInsertMode | Start in Insert mode instead of Normal Mode | Boolean | false |
  | vim.useCtrlKeys | Enable Vim ctrl keys overriding common VS Code operations such as copy, paste, find, etc. | Boolean | true |
  