Add logo to the readme file.

Author: Aleksi Salmela

README.md

@@ -4,6 +4,9 @@
 [![GitHub license](https://img.shields.io/badge/license-MIT-blue.svg)](https://raw.githubusercontent.com/tmc-cli/tmc-cli/master/LICENSE)
 [![Coverage Status](https://coveralls.io/repos/github/tmc-cli/tmc-cli/badge.svg?branch=master)](https://coveralls.io/github/tmc-cli/tmc-cli?branch=master)
 [![Build status](https://ci.appveyor.com/api/projects/status/np29sxc72y2f7d57?svg=true)](https://ci.appveyor.com/project/mikkomaa/tmc-cli)
+
+[![tmc-cli logo](https://rawgit.com/tmc-cli/tmc-cli/master/docs/logo_plain.svg)](logo_plain.svg)
+
 #tmc-cli
 
 TMC-CLI is the command-line client for University of Helsinki's Test My Code -framework. Test My Code is used by various online programming courses for exercise testing and submission.