chore: add PyPI badges + correct install command (#2)

Two consistency fixes on the landing repo so it matches what's now
live for the SDK:

- Added three badges at the top of the README (PyPI version, Python
versions, MIT license) pointing at the published socialapis-sdk
package. Used badge.fury.io for the version badge so Camo doesn't
serve a stale "not found" SVG from before the first publish.
- Swapped install instructions from `pip install socialapis` to
`pip install socialapis-sdk` in the README + examples/migrate.py.
The bare `socialapis` PyPI name was taken; the SDK ships as
socialapis-sdk. The Python import path stays `from socialapis
import ...` — unchanged.