Add regression issue template

.github/ISSUE_TEMPLATE/regression.md

@@ -0,0 +1,68 @@
+---
+name: Regression
+about: Report something that unexpectedly changed between Rust versions.
+labels: C-bug regression-untriaged
+---
+<!--
+Thank you for filing a regression report! 🐛 A regression is something that changed between versions of Rust but was not supposed to.
+
+Please provide a short summary of the regression, along with any information you feel is relevant to replicate it.
+-->
+
+### Code
+
+I tried this code:
+
+```rust
+<code>
+```
+
+I expected to see this happen: *explanation*
+
+Instead, this happened: *explanation*
+
+### Version it worked on
+
+<!--
+Provide the most recent version this worked on, for example:
+
+It most recently worked on: Rust 1.47
+-->
+
+It most recently worked on: <!-- version -->
+
+### Version with regression
+
+<!--
+Provide the version you are using that has the regression.
+-->
+
+`rustc --version --verbose`:
+```
+<version>
+```
+
+<!--
+Did the compiler crash? If so, please provide a backtrace.
+-->
+
+### Backtrace
+<!--
+Include a backtrace in the code block by setting `RUST_BACKTRACE=1` in your
+environment. E.g. `RUST_BACKTRACE=1 cargo build`.
+-->
+<details><summary>Backtrace</summary>
+<p>
+
+```
+<backtrace>
+```
+
+</p>
+</details>
+
+<!--
+If you know when this regression occurred, please add a line like below, replacing `{channel}` with one of stable, beta, or nightly.
+
+@rustbot modify labels: +regression-from-stable-to-{channel} -regression-untriaged
+-->