Need to update the readme file to install cartopy

[Harish-2003]

from gliderpy.fetchers import GliderDataFetcher
from matplotlib import pyplot as plt
from gliderpy.plotters import plot_track
glider_grab = GliderDataFetcher()

glider_grab.fetcher.dataset_id = "whoi_406-20160902T1700"
df = glider_grab.to_pandas()
df.head()
plot_track(df)

while writing this code it was showing an error like this

ModuleNotFoundError                       Traceback (most recent call last)
[<ipython-input-5-f1542db75b39>](https://localhost:8080/#) in <cell line: 3>()
      1 from gliderpy.fetchers import GliderDataFetcher
      2 from matplotlib import pyplot as plt
----> 3 from gliderpy.plotters import plot_track
      4 glider_grab = GliderDataFetcher()
      5 

1 frames
[/usr/local/lib/python3.10/dist-packages/gliderpy/plotters.py](https://localhost:8080/#) in <module>
      6 
      7 try:
----> 8     import cartopy.crs as ccrs
      9     import matplotlib.dates as mdates
     10     import matplotlib.pyplot as plt

ModuleNotFoundError: No module named 'cartopy'

and i think it would be better if we would update the readme file by adding

pip install cartopy

[ocefpaf]

@Harish-2003 I don't want to add any plotting instructions until the end of GSoC b/c:

1. the code is in fux
2. we wil change how we handle optional packages in gliderpy

With that said, let's leave this issue open to remind us to address this in the near future.

[ocefpaf]

We should make cartopy to be an optional dependency. Closing this one in favor of #117.