Singleton.md

+**Singleton** is a very well known and used design pattern.  
+  
+A singleton is a class that can be **instantiated only once**: this can be very useful when you want a single component to be in charge of all the interaction with a specific piece of hardware.
+
+In boardcore, you can achieve this by extending the Singleton template which you can find in `src/shared/`.
+
+### Interface
+The Singleton interface is a template class which exposes a single method:
+```cpp
+inline static T* getInstance()
+```
+What this method does is:
+- If an instance of this class already exists, return that instance.
+- If an instance of this class does not exist yet, create and return a new one.
+
+### Example
+
+From the `Singleton.h` comment:
+```cpp
+/**
+ * WARNING: deriving from this class is not enough to make a singleton,
+ * you also have to declare the constructor of your class private!
+ */
+class Foo : public Singleton<Foo>
+{
+  private:
+      Foo() {} // private constructor
+};
+```
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