Add issue template

ISSUE_TEMPLATE.md

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+### Explain the issue
+
+Most issues are related to bugs or problems. In these cases, you should include
+a minimal working example and a minimal vimrc file (see below), as well as:
+
+1. Steps to reproduce
+2. Expected behaviour
+3. Observed behaviour
+
+If your issue is instead a feature request or anything else, please consider if
+minimal examples and vimrc files might still be relevant.
+
+### Minimal working example
+
+Please provide a minimal working LaTeX example, e.g.
+
+```tex
+documentclass{minimal}
+\begin{document}
+
+Hello World!
+
+\end{document}
+```
+
+### Minimal vimrc file
+
+Please provide a minimal vimrc file that reproduces the issue. The following
+should often suffice:
+
+```vim
+set nocompatible
+
+" Load Vimtex
+let &rtp  = '~/.vim/bundle/vimtex,' . &rtp
+let &rtp .= ',~/.vim/bundle/vimtex/after'
+
+" Load other plugins, if necessary
+" let &rtp = '~/path/to/other/plugin,' . &rtp
+
+filetype plugin indent on
+syntax enable
+
+" Vimtex options go here
+```