Add description of sigproc fork

[gijzelaerr]

it would be very helpful to have a description of this work:

• Why is this fork created?
• What was missing/broken upstream?
• What is the relationship with other forks?
• What improvements have been made?
• If we want to sync with upstream, how do we do this? maybe with some example command.

[siemion]

Breakthrough Listen uses the filterbank format fairly extensively, and in ways far outside what was originally envisioned for the format. For instance, we frequently create filterbank files with 100s of million or even billions of channels, and many places in the standard sigproc the max number of channels is hardcoded to 16 or 32k. We also have built a number of tools to solve our particular problems (combining filterbank files, removing extraneous bits of the band, etc) that are most conveniently done linked against a modified version of sigproc. I don't think we want to sync this with upstream, I think we simply maintain it ourselves. Perhaps we could rename it to "bl_filterbank" which might be more accurate.