Link to drought.gov in README is out of date

[derrickburt]

Minor documentation issue in the README:

The link on drought.gov indices is deprecated. The new link is https://www.drought.gov/data-maps-tools/climate-and-drought-indices-python-spi-spei-pet

old (README.md)

This is a developmental/forked version of code that was originally developed by NIDIS/NCEI/NOAA. 
See [drought.gov](https://www.drought.gov/drought/python-climate-indices).

new (README.md)

This is a developmental/forked version of code that was originally developed by NIDIS/NCEI/NOAA. 
See [drought.gov][(https://www.drought.gov/drought/python-climate-indices](https://www.drought.gov/data-maps-tools/climate-and-drought-indices-python-spi-spei-pet).

I can update this link + submit a pull request, just need to fetch some upstream changes on my forked branch first and may take a minute before I can get to that.

Thanks!

[monocongo]

Thank you, @derrickburt ! Yes, thanks for this contribution. I don't have much time to maintain this project these days, every bit helps.