Time to `pull` from `rtl_433` repository. Revise docs now that omni decoder is included in rtl_433 repository

If you've been using a custom-built rtl_433 in order to use an Omnisensor microcontroller, you can now pull in your cloned repository and deploy a fully-updated version of rtl_433 with all current drivers and with omni included (now protocol #377).

Be sure to save a copy of your customized omni.c code so you can reinstall it before building rtl_433

The revisions in this "Omnisensor" release are just to simplify the documentation. Since omni.c has been incorporated into the rtl_433 repository as device protocol #377, these docs no longer need to describe how to build your own decoder into rtl_433.