Add a user guide at docs/users-guide.md #159

The df12 reference library at /docs lists every public repository that has a
concrete implementation and ships a docs/users-guide.md (or
docs/usage-guide.md). comenq qualifies on implementation but carries no user
guide, so it is currently excluded from the docs index.

Add a docs/users-guide.md covering at least:

• installation,
• configuration,
• day-to-day usage of the daemon and client.

Structure note: the docs generator renders each second-level (##) section of
the guide as a separate page in the generated bundle, so favour a flat
structure of ## sections with meaningful headings.

Once the guide lands on main, comenq can be added to the Tools section of
the reference library.