The 'enchant' C library was not found and maybe needs to be installed

After installing the prerequisites and running `pip`, I ran `make autobuild`. However, I saw the following error:

```
sphinx-autobuild -b dirhtml "docs" "docs/_build/html"
[sphinx-autobuild] > sphinx-build -b dirhtml getodk/docs/docs getodk/docs/docs/_build/html
Running Sphinx v5.3.0

Extension error:
Could not import extension sphinxcontrib.spelling (exception: The 'enchant' C library was not found and maybe needs to be installed.
See https://pyenchant.github.io/pyenchant/install.html
for details
)
Command exited with exit code: 2
The server will continue serving the build folder, but the contents being served are no longer in sync with the documentation sources. Please fix the cause of the error above or press Ctrl+C to stop the server.
```

After I followed the install instructions on the linked page, `make autobuild` ran successfully. I think it would be useful to document this step in the readme.

Start with the project README and the make autobuild target. Reproduce the documented build failure if possible, then add the required enchant C library installation step with the existing pyenchant instructions as reference. Run make autobuild to confirm the documentation build succeeds.

Written by the indexing model from the issue text.