added sqlite docs PoC

[roblaszczak]

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Motivation / Background

Detail

Alternative approaches considered (if applicable)

Checklist

The resources of our team are limited. There are a couple of things that you can do to help us merge your PR faster:

• [ ] I wrote tests for the changes.
• [ ] All tests are passing.
  • If you are testing a Pub/Sub, you can start Docker with make up.
  • You can start with make test_short for a quick check.
  • If you want to run all tests, use make test.
• [ ] Code has no breaking changes.
• [ ] (If applicable) documentation on watermill.io is updated.
  • Documentation is built in the github.com/ThreeDotsLabs/watermill/docs.
  • You can find development instructions in the DEVELOP.md.