feat: add error docs for E0423

docs/errors/E0423.md

@@ -0,0 +1,29 @@
+# E0423: missing fallthrough comment in switch case
+
+Switch Cases in javascript fallthrough to the next case if the `break` statement is not added at the end of the case.
+Since there is no explicit way of communication whether the fallthrough is intentional or not, it is recommended to use a comment indicating fallthrough.
+
+```javascript
+function test (c) {
+  switch (c) {
+    case 1:
+      foo();
+    default:
+      bar();
+  }
+}
+```
+
+To fix this error, place a comment at the end of `case 1` indicating fallthrough
+
+```javascript
+function test (c) {
+  switch (c) {
+    case 1:
+      foo();
+      //fallthrough
+    default:
+      bar();
+  }
+}
+```