Install from @latest, retract v0.0.1, print a go-installed binary's version (closes #116)

README "Install" now leads with go install ...@latest: until 1.0 each
release is a v0.0.N tag on main's tip, so @latest gives that tip once
it has passed the build. It also says what @main and v0.0.1 give and
how to check an installed binary. go.mod retracts v0.0.1, the early
build without a database.

sfdupes --version printed "dev" for every go-installed binary, since
only the linker set Version. When the linker sets none, it now prints
the main module's version from runtime/debug.ReadBuildInfo; "dev" is
left only when neither is known. Version is now empty by default.

The retraction takes effect only once a release carrying it is the
newest.

Model: opus-5-5

README.md

@@ -76,12 +76,13 @@ go version -m "$(command -v sfdupes)"
 ```
 
 `sfdupes --version` prints `sfdupes VERSION`. For a binary from `go install`,
-`VERSION` is the version Go recorded in it: the release tag, such as `v0.0.2`,
+`VERSION` is the version Go recorded in it: the release tag, such as `v0.0.3`,
 or for `@main` a pseudo-version ending in the first 12 characters of the
 commit's hash. A binary built from a clone or into the Docker image carries the
 git tag or commit it was built from instead, and `VERSION` is `dev` only when
 the binary carries no version at all. `go version -m` prints what Go recorded in
-the binary; its `mod` line names the module's version.
+the binary; its `mod` line names the module's version. Binaries from `v0.0.2`
+and earlier print `dev`, so check those with `go version -m`.
 
 From a clone, `make build` writes the binary to `./sfdupes`: