Paperai

Minor README update to note package can be installed from PyPI

Release addresses the following:

- Allow customized the QA model used for QA extraction (5)
- Migrated embeddings index logic to txtai project (7)

Release addresses the following:

- Add wildcard report queries (1) - Add ability to run report against entire database. This is only practical for smaller datasets.
- Fix Windows install issues (2)
- Embeddings index memory improvements (3) - Various improvements to limit memory usage when building an embeddings index
- Support must clauses for custom query columns (4) - Add same logic already present in general queries to require a term to be present when deriving report query columns

Initial release of paperai, migrating AI/ML/Search logic from existing [cord19q](https://github.com/neuml/cord19q) project.