Update doc regarding default static builds

[michikommader]

With the recent commits, the default building behaviour has changed from a shared to a static library which might be worth mentioning in the building hints.

Connected thoughts:

1. It could be practical to add a new version tag 0.6.0 for the changed default CMake building configuration. Reason: Some depending build chains will require reconfiguration if they have been linking libOpenDRIVE as shared lib without setting the according CMake variable explicitly.
2. Regarding the current Git version tag 0.5.0 I see that it is not in sync with the CMake project version which still states being 0.3.0. If libOpenDRIVE serves as CMake dependency in other software, it will be of relevance sooner or later. https://github.com/pageldev/libOpenDRIVE/blob/9a0437f8a18d445d5c43fe2a4c9401d8a4b770f0/CMakeLists.txt#L7