docs: add guide for reproducing issues

documentation/reproducing-issues.md

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+# Reproducing issues
+
+Since Neovim is very configurable and extensible, it's possible that some issues
+are caused by plugins or configuration. This can also make it very difficult to
+provide a fix. To help us diagnose the issue, please follow these steps:
+
+1. copy [repro.lua](../repro.lua) to a directory on your system
+2. run `nvim -u repro.lua` in that directory
+   - this will start Neovim with a minimal configuration, completely defined by
+     that one file.
+3. try to reproduce the issue
+   - if you can't reproduce the issue, make changes to the file to make it more
+     similar to your configuration until you can reproduce it.
+4. when reporting the issue, attach the contents of your `repro.lua` file to the
+   issue report.