Update README - Githubissues

The repo README is outdated and doesn't accurately reflect its current state. The README should:

Capture the definition of the SOMA API
Provide links to useful resources for developers (e.g. abstract_specification.md) and consumers (none as far as I known)
Provide information and links for tiledbsoma
Provide information and links for cell-census
Explain the intend to be a useful resource for single-cell data.

single-cell-data / SOMA