Main README does not properly build examples

Following the directions on the main README

### Building the examples

Unlike the applications, the examples must be compiled as a second step.
From your top corsika directory, (the one that includes `corsika-build` and `corsika-install`) run
```shell
cmake -Dcorsika_DIR=$PWD/corsika-build -S ./corsika/examples -B ./corsika-build-examples
cd corsika-build-examples
make -j4 #The number should match the number of available cores on your machine
```

emits a bunch of errors like

-- Configuring done
CMake Error at CMakeLists.txt:29 (add_executable):
  Target "em_shower" links to target "BZip2::BZip2" but the target was not
  found.  Perhaps a find_package() call is missing for an IMPORTED target, or
  an ALIAS target is missing?


CMake Error at CMakeLists.txt:29 (add_executable):
  Target "em_shower" links to target "Boost::filesystem" but the target was
  not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?

I guess this requires some updating for CMakeLists.txt