add cmake doc

doc/cmake.md

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+# Building a new data model library with CMake
+
+PODIO exposes multiple CMake macros to enable the building of data model libraries.
+For a package developer tutorial on CMake, please have a look [here])https://hsf-training.github.io/hsf-training-cmake-webpage/). PODIO follows the same conventions used therein.
+
+## Generate the C++ code
+After a proper `find_package(PODIO)`, the C++ code of the new data model `NewDM` can be created and compiled via
+
+```
+# generate the c++ code from the yaml definition
+PODIO_GENERATE_DATAMODEL(newdm newdm.yaml headers sources IO_BACKEND_HANDLERS)
+
+# compile the core data model shared library (no I/O)
+PODIO_ADD_DATAMODEL_CORE_LIB(newdm "${headers}" "${sources}")
+```
+
+However, this does not create any I/O components yet. For this, one needs to add the `IO_BACKEND_HANDLERS` parameter and then generate and compile the corresponding backends, like shown below for the ROOT and SIO backends:
+
+```
+# generate the c++ code from the yaml definition
+PODIO_GENERATE_DATAMODEL(newdm newdm.yaml headers sources IO_BACKEND_HANDLERS "ROOT;SIO")
+
+# compile the core data model shared library (no I/O)
+PODIO_ADD_DATAMODEL_CORE_LIB(newdm "${headers}" "${sources}")
+
+# generate and compile the ROOT I/O dictionary
+PODIO_ADD_ROOT_IO_DICT(newdmDict newmdm "${headers}" src/selection.xml)
+
+# compile the SIOBlocks shared library for the SIO backend
+PODIO_ADD_SIO_IO_BLOCKS(edm4hep "${headers}" "${sources}")
+```
+
+For a more detailed example, please have a look at [EDM4hep](https://github.com/key4hep/EDM4hep/blob/main/edm4hep/CMakeLists.txt)