Add README to describe usage of serialized source

examples/serialized_custom_source/README.md

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+# Building a Serialised Custom Source
+
+To run this example, you first need to compile the custom source library, which
+requires headers from OpenMC. A CMakeLists.txt file has been set up for you that
+will search for OpenMC and build the custom library. To build the source
+library, you can run:
+
+    mkdir build && cd build
+    OPENMC_ROOT=<path_to_openmc_install> cmake ..
+    make
+
+After this, you can build the model by running `python build_xml.py`. In the XML
+files that are created, you should see a reference to build/libserialised_source.so,
+the custom source library that was built by CMake. The model is also set up with a
+mesh tally of the flux, so once you run `openmc`, you will get a statepoint file
+with the tally results in it. Running `python show_flux.py` will pull in the
+results from the statepoint file and display them. If all worked well, you
+should see a ring "imprint" as well as a higher flux to the right side (since
+the custom source has all particles moving in the positive x direction).
+
+Once built, you can edit the serialised_source.xml file to change the radius of the
+sampled ring or the energy of the sampled particles.