add cmake_prefix and DIR reminder for cmake example

Author: dtotsila

src/docs/docs/faq.md

@@ -406,6 +406,17 @@ In C++ you are also able to pre-allocate a custom number of OpenGL contexts so t
 
 You need to use `waf` to build RobotDART, but when installing the library a CMake module is installed. Thus it is possible use RobotDART in your code using CMake. You can find a complete example at [cmake/example](https://github.com/resibots/robot_dart/blob/master/cmake/example). In short the CMake would look like this:
 
+### Prerequisites:
+- Ensure `robot_dart` is installed.
+- If `robot_dart` and `magnum` are not installed in standard locations, you can either:
+  - Set the `CMAKE_PREFIX_PATH` environment variable:
+    ```sh
+    export CMAKE_PREFIX_PATH=/opt/robot_dart:/opt/magnum
+    ```
+  - Or, specify the paths directly when running `cmake`:
+    ```sh
+    cmake -DRobotDART_DIR=/opt/robot_dart -DMagnum_DIR=/opt/magnum ..
+    ```
 ``` cmake
 cmake_minimum_required(VERSION 3.10 FATAL_ERROR)
 project(robot_dart_example)