Add a new issue template for code improvements (rust-lang#718)

Co-authored-by: Adrian Palacios <[email protected]>

Author: 2 people

.github/ISSUE_TEMPLATE/code_improvement.md

@@ -0,0 +1,28 @@
+---
+name: Code Improvement
+about: Create a request for improving the code quality of RMC
+labels: Cat: cleanup, Cat: enhancement 
+---
+<!--
+Please report security concerns to us on the
+[AWS vulnerability reporting](http://aws.amazon.com/security/vulnerability-reporting/) page.
+-->
+
+<!--
+Thank you for requesting an RMC improvement!
+Please include the following information if it applies to your case:
+-->
+**Requested improvement:**
+
+**Related features:**
+
+**Is this a breaking change?**
+
+**Target code:**
+<!--
+Either paste a snippet with code that requires improvement or paste a link to
+the code that this will target.
+-->
+```rust
+<code>
+```