Add more topics / references and a informative on type of reference

README.md

@@ -3,10 +3,24 @@
 
 <img src="resources/topic-graph.png" width="100%">
 
+### More info
+> Information about the links
+
+* 📕 - Book
+* 👩‍💻 - Project
+* 🎓 - Online Course
+* 🆓 - No money required
+* 💰 - Paid
+
+## References
+
 * Site Reliability Engineering
   * Software Engineering
     * Programming Languages
       * Functional Programming Languages
-        * [Haskell From First Principles](https://github.com/marceloboeira/hffp)
+        * [Haskell From First Principles](https://github.com/marceloboeira/hffp) 📕 💰
       * Systems Programming Languages
-        * [The Rust Programming Language](https://github.com/marceloboeira/trpl)
+        * [The Rust Programming Language](https://github.com/marceloboeira/trpl) 📕 🆓
+  * Operations
+    * Operating Systems
+      * [Introduction to Operating Systems](https://eu.udacity.com/course/introduction-to-operating-systems--ud923) 🎓 🆓