Create bug.md

.github/ISSUE_TEMPLATES/bug.md

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+---
+name: Bug report
+about: Create a bug report
+title: "[BUG]"
+labels: bug
+assignees: ''
+
+---
+
+Please search existing issues to avoid duplicates.
+
+## Details
+
+
+### Backends
+
+This bug occurs on ( insert `x` below )
+
+Backend      | yes | no  | untested
+-------------|-----|-----|---------
+gr (default) |     |     |
+pyplot       |     |     |
+plotly       |     |     |
+plotlyjs     |     |     |
+pgfplotsx    |     |     |
+
+### Versions
+
+Plots.jl version:
+Backend  version:
+Output of `versioninfo()`: