README update and missing dependencies.

vendor/github.com/pztrn/mogrus/doc/logging_to_mattermost.md

@@ -0,0 +1,55 @@
+This example presumes:
+
+1. You're using mogrus as logger.
+
+2. You want to log messages from something to Mattermost.
+
+3. You have ``SendMessage(channelID string, msg string)`` in your Mattermost's
+client library for sending messages to desired channel.
+
+4. ``cl.DebugChannelID is`` a channel ID to which mogrus will log.
+
+Here's io.Writer-compatible struct for using with mogrus:
+
+```
+var logMessage = `Level: %s
+Timestamp: %s
+ 
+    %s`
+ 
+// MattermostChannelLogger provides io.Writer compatible interface for
+// writing logs to Mattermost channel.
+type MattermostChannelLogger struct{}
+ 
+func (mcl MattermostChannelLogger) Write(p []byte) (n int, err error) {
+    // Generate eye-candy message.
+    pstr := string(p)
+ 
+    timer := regexp.MustCompile("time=\"([0-9. -:]+)\"")
+    timeFound := timer.FindAllStringSubmatch(pstr, -1)
+    time := timeFound[0][1]
+ 
+    levelr := regexp.MustCompile("level=([a-zA-Z]+)")
+    levelFound := levelr.FindAllStringSubmatch(pstr, -1)
+    level := levelFound[0][1]
+ 
+    msgr := regexp.MustCompile("msg=\"(.+)\"")
+    msgFound := msgr.FindAllStringSubmatch(pstr, -1)
+    logmsg := msgFound[0][1]
+ 
+    msg := fmt.Sprintf(logMessage, level, time, logmsg)
+ 
+    cl.SendMessage(cl.DebugChannelID, msg)
+ 
+    return len(p), nil
+}
+```
+
+Initialize it after connecting to Mattermost server like:
+
+```
+// Register output for logging into Mattermost's channel.
+c.Log.CreateOutput("Mattermost debug channel", MattermostChannelLogger{}, false, cfg.System.Logging.Level)
+```
+
+Where ``c.Log`` is a mogrus instance, ``cfg.System.Logging.Level`` is one of logging levels ("debug", "info", "warn" or "error").