Add failing example for E0170 explanation

src/librustc_error_codes/error_codes/E0170.md

@@ -1,3 +1,24 @@
+A pattern binding is using the same name as one of the variants a type.
+
+Erroneous code example:
+
+```compile_fail,E0170
+# #![deny(warnings)]
+enum Method {
+    GET,
+    POST,
+}
+
+fn is_empty(s: Method) -> bool {
+    match s {
+        GET => true,
+        _ => false
+    }
+}
+
+fn main() {}
+```
+
 Enum variants are qualified by default. For example, given this type:
 
 ```