add github badges

README.md

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 # Lantern Extras
 
+[![build](https://github.com/lanterndata/lantern_extras/actions/workflows/build.yaml/badge.svg?branch=main)](https://github.com/lanterndata/lantern_extras/actions/workflows/build.yaml)
+[![codecov](https://codecov.io/github/lanterndata/lantern_extras/branch/main/graph/badge.svg)](https://codecov.io/github/lanterndata/lantern_extras)
+[![test](https://github.com/lanterndata/lantern_extras/actions/workflows/test.yaml/badge.svg?branch=main)](https://github.com/lanterndata/lantern_extras/actions/workflows/test.yaml)
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 This extension makes it easy to experiment with embeddings from inside a Postgres database. We use this extension along with [Lantern](https://github.com/lanterndata/lantern) to make vector operations performant. But all the helpers here are standalone and may be used without the main database.
 
 **NOTE**: Functions defined in this extension use Postgres in ways Postgres is usually not used.