Document the previous domain gathering process

[gbinal]

Make sure to write how pulse assembled the target URL list it would use. Write down here.

[gbinal]

Notes to self:

• Domain-scan uses this to operate the overall gathering process.

• Pulse would specific which gatherers to run here.

• And the location of the data sources to be gathered was here.

• (In an earlier era, when censys and rdns data was pulled live instead of from a static snapshot, the code to do that was here.)

[gbinal]

First pass written here.