adding a README; wip: needs updating w/ better docs
adding a README; wip: needs updating w/ better docs

tndatacommons/tndata_backend,izzyalonso/tndata_backend,tndatacommons/tndata_backend,tndatacommons/tndata_backend,izzyalonso/tndata_backend,izzyalonso/tndata_backend,tndatacommons/tndata_backend,izzyalonso/tndata_backend