Added missing_converter.md issue template (#160)

Author: Josh Goldberg

.github/ISSUE_TEMPLATE/missing_converter.md

@@ -0,0 +1,24 @@
+---
+name: Missing Converter
+about: Report a missing TSLint-to-ESLint rule converter in tslint-to-eslint-config
+---
+
+<!--
+👋 Hi, thanks for filing an issue on tslint-to-eslint-config! 💖
+Please fill out all fields below to ensure your issue is addressed.
+
+If your issue doesn't provide enough info to fully explain or reproduce your bug, it will be closed. 😦
+-->
+
+### 💥 Missing Converter
+
+-   `tslint-to-eslint-config` version: X.X.X
+-   ESLint version: X.X.X
+
+#### TSLint Rule Name
+
+<!-- Which TSLint rule did tslint-to-eslint-config report not having a converter?  -->
+
+#### Package Source
+
+<!-- Optional: If not from TSLint core, which ruleset is the TSLint rule from? -->