Take the version from the git tag, not a constant (closes #80)

`mfer version`, `mfer --version` and the banner print the revision the
build stamps into main.Gitrev or, when it stamped none (as after
`go install` of a tag), the main module's version from
runtime/debug.ReadBuildInfo. The banner no longer shows a release date.

mfer.Version and mfer.ReleaseDate go, with main.Version and the version
field and parameter in internal/cli that nothing read. A program using
the library reads the module version from its own build information.
The README says where the version comes from.

Judgement call: a binary with no build information, or none holding a
module version, prints "unknown", the word script/build already uses
when git describe gives nothing.

Model: opus-5-5

README.md

@@ -17,6 +17,15 @@ The current version is pre-1.0 and while the repo was published in 2022, there
 has not yet been any versioned release. [SemVer](https://semver.org) will be
 used for releases.
 
+A version is a git tag, and no file in the tree records a version number.
+`mfer version` prints the revision the build stamped into the binary, which for
+`make build` and the Docker image is the output of `git describe` on the
+checkout they build. A binary built without that stamp, as
+`go install sneak.berlin/go/mfer/cmd/mfer@<tag>` builds it, prints the module
+version Go records in it instead. A program that imports the `mfer` package
+finds the version it was built against in its own build information
+(`runtime/debug.ReadBuildInfo`).
+
 This project was started by [@sneak](https://sneak.berlin) to scratch an itch in
 2022 and is currently a one-person effort, though the goal is for this to emerge
 as a de-facto standard and be incorporated into other software. A compatible