Create plot_extraction.md

docs/plot_extraction.md

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+## Create a rectangular shapefile to extract area of interest from agricultural research plots
+
+
+**plantcv.geospatial.extract_plot**(*input_filename, output_filename, ranges, columns, length of row, vertical alley, horizontal alley)
+
+**returns** [Multi-Polygon Shapefile]
+
+- **Parameters:**
+    - input_filename - Filepath to input shapefile consiting four corners of the field
+    - output_filename - Filepath to output shapefile containing individual rectangular ploygon corresponding to specific research plot
+    - ranges - Number of ranges
+    - columns - Number of columns
+    - length of row - Row longth along column in meters
+    - vertical alley - Length of alley between ranges in meters
+    - horziontal alley - Length of alley between columns in meters
+
+- **Example use:**
+    - below
+
+
+```python
+import plantcv.plantcv as pcv 
+import plantcv.geospatial.extract_plot as gridcell
+
+# Path of input and output shapefile
+input_shapefile_path = '712_corner_points.shp'
+output_shapefile_path = '712_polygon.shp'  
+output_shapefile = gridcell.write_shapefile(input_shapefile_path, output_shapefile_path, number_of_ranges=45, number_of_columns=12, row_length_along_column=2.4384, vertical_alley=0.9144, horizontal_alley=0.03048)
+
+```
+
+**Source Code:** [Here](https://github.com/danforthcenter/plantcv-geospatial/blob/main/plantcv/geospatial/plot_extract.py)