Add user documentation

[svenkarol]

Somebody should add some documentation on how to get some of the examples running. I think of a short intro on how to generate code and how to compile and run a simulation. Also syntax should be explained somewhere.

[lschuetze]

Currently, the only way is to

1. install a local copy of OpenPME and prepare your MPS installation such that you can build the project and the examples (see README.md)
2. Given you opened an example, you can open the context and select "Preview Generated Text"
3. Copy the content of the opened file containing the generated code
4. Paste it into a file
5. Prepare a Makefile by looking into the OpenFPM examples
6. Install OpenFPM and execute the project

[kraatz]

Step 2-4 can be combined if you select "Rebuild model" in the context menu of the model containing the example. This generates an output file in the solutions folder of the project.