Update README.md -> pass USE_REALSENSE through pip

Author: MedericFourmy

README.md

@@ -26,7 +26,6 @@ pip install .
 > M3T relies on [GLFW](https://www.glfw.org/). Before building ensure it is installed.
 > For Ubuntu run `apt-get install libglfw3 libglfw3-dev`
 
-
 ```
 git clone https://github.com/agimus-project/pym3t
 cd pym3t
@@ -37,6 +36,11 @@ pip install .
 pip install .[examples]
 ```
 
+### Building with realsense support
+First ensure that [librealsense](https://github.com/IntelRealSense/librealsense) is installed. It should be the case if you followed the conda-based installation. Then build with:
+
+`pip install . --config-settings="cmake.define.USE_REALSENSE=ON"`
+
 # Example scripts
 As example usage of the library, we provide several scripts: 
 * `run_image_dir_example.py`: single object tracking using color and depth images from filesystem;
@@ -97,4 +101,4 @@ python examples/run_realsense_example.py --use_region --use_depth -b obj_000014
 Keyboard commands:
 - `q`: exit;
 - `d`: initialize object pose;
-- `x`: start/restart tracking.
+- `x`: start/restart tracking.