add documentation on how to use uniqueidgensvc (#84)

Author: fdplacido

Diff for: doc/uniqueIDGen.md

@@ -0,0 +1,35 @@
+# How to use UniqueIDGenSvc
+
+A Service to generate unique identifiers can be used with the following inputs:
+- Event number, Run number and Algorithm name
+- Seed number: set in the options python file
+
+To set a seed for the UniqueIDGenSvc, add the following to an options file:
+
+```python
+from Configurables import UniqueIDGenSvc
+UniqueIDGenSvc().Seed = 987654321
+```
+
+Declare the service in the header file:
+
+```cpp
+#include <k4FWCore/IUniqueIDGenSvc.h>
+
+SmartIF<IUniqueIDGenSvc> m_service;
+```
+
+Initialize the service:
+
+```cpp
+StatusCode SomeGaudiAlgorithm::initialize() {
+  m_service = serviceLocator()->service("UniqueIDGenSvc");
+```
+
+Then use the service during execution:
+
+```cpp
+StatusCode MarlinProcessorWrapper::execute() {
+  m_service->getUniqueID(1, 2, name());
+}
+```