chore(github): add issue template (#2755)

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,15 @@
+<!-- BUGS: Please use this template. -->
+
+**Prettier Version:** 1.6.1 / master
+
+**Code**
+
+([playground](https://prettier.io/playground/#.....))
+
+```js
+// code snippet
+```
+
+**Expected behavior:**
+
+**Actual behavior:**