[DOCS] add tutorial documentation in COLOR_THRESHOLD.md

docs/COLOR_THRESHOLD.md

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+## Color-Segmenting Objects to Use with TrackDLO
+
+TrackDLO uses color thresholding to obtain the DLO segmentation mask. The [`color_picker.py`](https://github.com/RMDLO/trackdlo/blob/master/utils/color_picker.py) script is provided to simplify tuning the color threshold values using a simple interface with a track bar. This document provides a tutorial of intended use for this script. 
+
+First, save an image to use for color thresholding. We use an Intel RealSense d435 camera. The RealSense ROS package publishes raw RGB images to the `/camera/color/image_raw` topic. Raw `.png` images can be extracted from this topic using the [image_view](http://wiki.ros.org/image_view#image_view.2Fdiamondback.image_saver) ROS package:
+
+Terminal 1:
+
+```
+roslaunch trackdlo realsense_node.launch
+```
+
+Terminal 2:
+
+```
+rosrun image_view image_saver image:=/camera/color/image_raw
+```
+
+This is an example of a rope image ([`rope.png`](https://github.com/RMDLO/trackdlo/blob/master/images/rope.png)) from our camera:
+
+<p align="center">
+  <img src="../images/rope.png" width="450" title="RGB Image of a Blue Rope">
+</p>
+
+Next, run the user interface (replace `images/rope.png` with the relative or full path to the image of the DLO).
+
+```
+cd /path/to/trackdlo/root/folder && python utils/color_picker.py images/rope.png
+```
+
+The user interface should appear as shown below. Use the track bars to tune the HSV lower and upper limits to segment the background.
+
+<p align="center">
+  <img src="../images/thresholder.png" width="450" title="RGB Image of a Blue Rope">
+</p>
+
+Once the HSV threshold values are determined with this tool, modify the default values according to the instructions below:
+* Monochrome DLO: set the lower and upper HSV value ranges in the [`trackdlo.launch`](https://github.com/RMDLO/trackdlo/blob/master/launch/trackdlo.launch) file. 
+* Multi-Colored DLO: set `multi_color_dlo` to `true` in [`trackdlo.launch`](https://github.com/RMDLO/trackdlo/blob/master/launch/trackdlo.launch), then modify the `color_thresholding` function in [`initialize.py`](https://github.com/RMDLO/trackdlo/blob/master/trackdlo/src/initialize.py) and [`trackdlo_node.cpp`](https://github.com/RMDLO/trackdlo/blob/master/trackdlo/src/trackdlo_node.cpp) to customize DLO segmentation.