Work towards #56: Splitting up the README

I did the job and splitted up the readme, hopefully everything was
splitted correctly...

singleton/index.md

@@ -0,0 +1,27 @@
+---
+layout: pattern
+title: Singleton
+folder: singleton
+categories: pattern_cat
+tags: pattern_tag
+---
+
+**Intent:** Ensure a class only has one instance, and provide a global point of
+access to it.
+
+![alt text](./etc/singleton_1.png "Singleton")
+
+**Applicability:** Use the Singleton pattern when
+
+* there must be exactly one instance of a class, and it must be accessible to clients from a well-known access point
+* when the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code
+
+**Typical Use Case:**
+
+* the logging class
+* managing a connection to a database
+* file manager
+
+**Real world examples:**
+
+* [java.lang.Runtime#getRuntime()](http://docs.oracle.com/javase/8/docs/api/java/lang/Runtime.html#getRuntime%28%29)