Add brief description of new test case

test/README.md

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+# k4MarlinWrapper tests
+
+The test cases for the k4MarlinWrapper are somewhat involved in some cases.
+Hence, we try to describe their intent and their high level layout a bit here.
+We will be refering to the test cases by name as they are defined in the
+[`CMakeLists.txt`](./CMakeLists.txt) file in here.
+
+## `global_converter_maps`
+
+The main purpose of this test is to make sure that the EDM conversions work as
+expected, even if the event is converted piece-by-piece. In order to make that
+work the individual converter tool instances have to share a mapping of all
+corresponding LCIO and EDM4hep objects that have been converted (in either
+direction). This map is referred to as the *global object map*.
+
+### Setup
+
+The test is using the
+[`global_converter_maps.sh`](./scripts/global_converter_maps.sh) script which
+effectively simply runs the
+[`test_global_converter_maps.py`](./gaudi_opts/test_global_converter_maps.py)
+options file after downloading some input data. This options file uses two
+wrapped MarlinProcessors and one GaudiAlgorithm plus some converters inbetween
+them:
+- `PodioInput` to read the *MCParticles* collection from the input file (in
+  EDM4hep format)
+- [`PseudoRecoProcessor`](./src/PseudoRecoProcessor.cc) creates a reco particle
+  for every MC particle in the input collection
+  - An EDM4hep to LCIO converter converts the input MC particles up front
+  - An LCIO to EDM4hep converter converts the output reco particles afterwards
+- [`TrivalMCTruthLinkerProcessor`](./src/TrivialMCTruthLinkerProcessor.cc)
+  creates trivial links from the MC particles to the reco particles.
+  - An LCIO to EDM4hep converter converts the output collction to EDM4hep
+- [`MCRecoLinkChecker`](./src/MCRecoLinkChecker.cc) is a Gaudi algorithm that
+  simply checks whether the MC-reco links are as expected.
+
+Without the global converter map, the second LCIO to EDM4hep converter that is
+run after the MC - reco linker will not produce valid MC - reco links, as it
+does not have any knowledge about the input MCs and recos.