feat(extras): Add README.md

extras/README.md

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+Extras
+======
+
+## Showcases
+
+The directory `showcase` includes files for programming languages. The idea
+is to have showcases to check treesitter or LSP support highlights all the
+things correctly. If something is not working probably for a language please
+contribute a showcase file!
+
+## Reproducer
+
+In order to report bugs there is a simple lua config to create a reproducer.
+This makes it easier to fix bugs or add new features.
+
+Note that the `on_highlights` callback can be used to implement support for new
+plugins and you can provide a reproducer to show how it should look like.
+
+First download the `repro.lua` file
+[here](https://raw.githubusercontent.com/cryptomilk/nightcity.nvim/main/extras/repro.lua).
+Modify it and run Neovim with:
+
+    nvim -u repro.lua
+
+Once you've reproduced the bug or added the needed stuff to `on_highlights` zip
+the `repro.lua` and `.repro` directory and upload it.