add a README covering usage, theory, architecture and limits

[rmarable]

The repository had no documentation. This adds one file.

• The CLI surface: the usage text verbatim, since there is no --help;
  the five operations with the SQL each one builds; and every option
  with its actual default.
• The theory: addressing a row by key is addressing a file by name, and
  why single-key whole-value access is the pattern that scales.
• The architecture: the two data paths through get() and put(), the
  mmap-or-spool decision, and the temp-file-and-rename output.
• The limits: the size ceiling, the mmap snapshot hazard, what select
  does and does not preserve when it replaces a file, and the rest.
• The build dependency: libpq, with the package name for macOS, Debian
  and RHEL, since pg_config is on none of them by default.

Verified against a throwaway PostgreSQL 18.6 cluster, 36 checks:

• The documented examples, with cmp proving the awk round trip is
  byte-identical, and the same for the tar pipe.
• Every operation and its failure mode: missing key, multiple rows, NULL
  value, duplicate insert, replace and append against no row, append
  over NULL.
• integer, inet and uuid key columns.
• Both -M forms: to_timestamp(N) for a regular file, 'now'::TIMESTAMP
  for a pipe.
• File behaviour: an existing target keeps its mode, a fresh retrieval
  of a 755 executable lands at 644, no trailing newline into a file, and
  a failed select leaves the target byte-identical with no temp file
  left behind.

The second commit is what that testing changed. The first draft said the
write direction of -b was unchecked. In fact the server refuses a write
to a bytea column without -b as a type mismatch, so only one of the four
combinations is silent: a write to a text column with -b, which stores
the hex form at twice the file's length plus two.

Two claims that had rested on the documented cast rules are now observed.
-b into a text column stores hex, and a text column rejects a zero byte
with invalid byte sequence for encoding "UTF8": 0x00.

The third commit is wording only.

Not covered: the roughly 1 GB size ceiling, and the /proc behaviour,
since the testing ran on macOS.

🤖 Generated with Claude Code

[vixie]

the program is a work in progress, and the proposed readme is about to be out of date. i'll leave this pr open for now.