some minimal documentation for Logger.Writer{,Level}

This also includes two examples extracted from the readme.

writer_test.go

@@ -0,0 +1,34 @@
+package logrus_test
+
+import (
+	"log"
+	"net/http"
+
+	"github.com/sirupsen/logrus"
+)
+
+func ExampleLogger_Writer_httpServer() {
+	logger := logrus.New()
+	w := logger.Writer()
+	defer w.Close()
+
+	srv := http.Server{
+		// create a stdlib log.Logger that writes to
+		// logrus.Logger.
+		ErrorLog: log.New(w, "", 0),
+	}
+
+	if err := srv.ListenAndServe(); err != nil {
+		logger.Fatal(err)
+	}
+}
+
+func ExampleLogger_Writer_stdlib() {
+	logger := logrus.New()
+	logger.Formatter = &logrus.JSONFormatter{}
+
+	// Use logrus for standard log output
+	// Note that `log` here references stdlib's log
+	// Not logrus imported under the name `log`.
+	log.SetOutput(logger.Writer())
+}