Provide documentation for "ignored" rules #409

We need to document the internal rules from SCTK and SCIO that determine what files are reported as "uninteresting" or not reported at all. Two examples are:

• ignore.py in commoncode: https://github.com/nexB/commoncode/blob/653225d58dcf5d1f75dbe572cac13f6d84ae755b/src/commoncode/ignore.py#L63 has a long list of files ignored (not reported) by SCTK.
• rootfs.py in scancode.io: https://github.com/nexB/scancode.io/blob/main/scanpipe/pipes/rootfs.py has a list of rootfs directories that we report, but label as "ignored-not-interesting".

In both cases the most important thing is to document the existence of these rules and the second priority is to document the rules themselves. We need to start by updating the SCIO documentation to explain why we have these rules and provide a link to where they are in the code. Longer term we need to figure out how to automate documentation of rules within our software.