Added markdown file for rgb2cmyk function

docs/rgb2cmyk.md

@@ -0,0 +1,77 @@
+## RGB to CMYK
+
+Convert image from RGB color space to CMYK color space and split the channels.
+
+**plantcv.rgb2gray_cmyk**(*rgb_img, channel*)
+
+**returns** split image (c, m, y or k channel)
+
+- **Parameters:**
+    - rgb_img - RGB image data
+    - channel - Split 'c' (cyan), 'm' (magenta), 'y' (yellow) or 'k' (black) channel
+
+- **Context:**
+    - Used to help differentiate plant and background
+
+
+**Original RGB image**
+
+![Screenshot](img/documentation_images/rgb2cmyk/original_image.jpg)
+
+```python
+
+from plantcv import plantcv as pcv
+
+# Set global debug behavior to None (default), "print" (to file), 
+# or "plot" (Jupyter Notebooks or X11)
+pcv.params.debug = "print"
+
+# image converted from RGB to LAB, channels are then split. 
+# Lightness ('l') channel is output
+l_channel = pcv.rgb2gray_cmyk(rgb_img=rgb_img, channel='Y')
+
+```
+
+**Cyan channel image**
+
+![Screenshot](img/documentation_images/rgb2lab/CMYK-cyan.jpg)
+
+```python
+
+# Green-Magenta ('a') channel is output
+a_channel = pcv.rgb2gray_cmyk(rgb_img=rgb_img, channel='C')
+
+```
+
+**Magenta channel image**
+
+![Screenshot](img/documentation_images/rgb2lab/CMYK-magenta.jpg)
+
+```python
+
+# Green-Magenta ('a') channel is output
+a_channel = pcv.rgb2gray_cmyk(rgb_img=rgb_img, channel='M')
+
+```
+
+**Yellow channel image**
+
+![Screenshot](img/documentation_images/rgb2lab/CMYK-yellow.jpg)
+
+```python
+
+# Green-Magenta ('a') channel is output
+a_channel = pcv.rgb2gray_cmyk(rgb_img=rgb_img, channel='Y')
+
+```
+
+**Black channel image**
+
+![Screenshot](img/documentation_images/rgb2lab/CMYK-black.jpg)
+
+```python
+
+# Green-Magenta ('a') channel is output
+a_channel = pcv.rgb2gray_cmyk(rgb_img=rgb_img, channel='K')
+
+```