Add A-diagnostics bug report template

.github/ISSUE_TEMPLATE/diagnostics.md

@@ -0,0 +1,46 @@
+---
+name: Diagnostic issue
+about: Create a bug report or feature request for a change to `rustc`'s error output
+labels: A-diagnostics, T-compiler
+---
+<!--
+Thank you for filing a bug report! 🐛 Please provide a short summary of the bug,
+along with any information you feel relevant to replicating the bug.
+
+If you cannot produce a minimal reproduction case (something that would work in
+isolation), please provide the steps or even link to a repository that causes
+the problematic output to occur. 
+-->
+
+Given the following code: <!-- Please provide a link to play.rust-lang.org -->
+
+```rust
+<code>
+```
+
+The current output is:
+
+```
+<rustc output>
+```
+
+<!-- The following is not always necessary. -->
+Ideally the output should look like:
+
+```
+<proposed output>
+```
+
+<!--
+If the problem is not self-explanatory, please provide a rationale for the
+change.
+-->
+
+<!--
+If dramatically different output is caused by small changes, consider also
+adding them here.
+
+If you're using the stable version of the compiler, you should also check if the
+bug also exists in the beta or nightly versions. The output might also be
+different depending on the Edition.
+-->