fix developer guide url

[Bud-Macaulay]

URL to the developer guide in the readme is broken,

not sure how changing URLs are handled for the docs.
If you would rather fix this with a redirect elsewhere feel free to close.

[mbercx]

Right, I moved that in 1b80920, nice catch, thanks for the fix @Bud-Macaulay!

[mbercx]

Docs build fails because of this warning, I think. Will fix in a bit:

WARNING: failed to reach any of the inventories with the following issues:
intersphinx inventory 'https://pymatgen.org/objects.inv' not fetchable due to <class 'requests.exceptions.HTTPError'>: 404 Client Error: Not Found for url: https://pymatgen.org/objects.inv

[mbercx]

If you would rather fix this with a redirect elsewhere feel free to close.

We don't do fancy stuff like that 'round here 😉