DOC: Clarify separate CudaCommon compilation in INSTALLATION.md

INSTALLATION.md

@@ -19,6 +19,7 @@ Independent configuration and compilation
 -----------------------------------------
 For RTK developpers, it may be useful to compile RTK independently from ITK. This is possible, simply:
 * Compile ITK with `Module_RTK=OFF`.
+* If you want to use CUDA, also activate `Module_CudaCommon` or compile it separately as RTK in the following two bullet points (cloning its [GitHub repository](https://github.com/RTKConsortium/ITKCudaCommon) or downloading it as a [zip package](https://codeload.github.com/RTKConsortium/ITKCudaCommon/zip/master)).
 * Manually download RTK's source repository from [GitHub](https://github.com/RTKConsortium/RTK) with `git` (recommended) or as a [zip package](https://codeload.github.com/RTKConsortium/RTK/zip/master).
 * Configure the project with CMake pointing to RTK's source directory and setting the CMake option `ITK_DIR` to ITK's compilation directory. All CMake options above can be set except `Module_RTK`.