docs: add limitation note for loading REPEATED fields

[plamut]

Closes #17.

This PR adds a limitation note about uploading REPEATED (i.e. array) fields with the load_table_from_dataframe() method.

It appears that REPEATED fields in parquet files (used for uploading the data) are encoded in a way that is not always compatible with the table schema definition on the backend.

PR checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)