write up rationale for .sig.zip files over JSON files

[ctb]

ref:

• https://github.com/sourmash-bio/sourmash_plugin_branchwater/issues/501
• discussion in https://github.com/sourmash-bio/sourmash_plugin_branchwater/pull/445
• and also benchmarks in https://github.com/sourmash-bio/sourmash_plugin_branchwater/issues/525

[ctb]

some short notes:

• random access to specific sketches, incl over HTTP;
• manifest for select, picklists;
• can be inspected with "standard" tools;