added doc.go for better rendering in godoc

doc.go

@@ -0,0 +1,21 @@
+/*
+Package problem implements RFC7807 `application/problem+json` and
+`application/problem+xml` using the functional options paradigm.
+
+# Features
+
+- Compatible with `application/problem+json`.
+- Inspired by https://github.com/zalando/problem.
+- RFC link https://tools.ietf.org/html/rfc7807.
+- A Problem implements the Error interface and can be compared with errors.Is().
+- Wrap an error to a Problem.
+- `application/problem+xml` is also supported using `xml.Unmarshal` and `xml.Marshal`.
+- Auto-Title based on StatusCode with `problem.Of(statusCode)`.
+
+# Installation
+
+To install the package, run:
+
+go get -u schneider.vip/problem
+*/
+package problem