Add Git-derived version reporting

README.md

@@ -131,6 +131,12 @@ The resulting executable is:
 build/kindle2notes
 ```
 
+When the source is built from Git, CMake obtains the version from
+`git describe`. A build made exactly from a release tag reports only that tag
+(for example, `1.0`). Later commits include their distance from the tag and
+the abbreviated commit hash (for example, `1.0-3-gabc1234`). A modified
+working tree also receives the `-dirty` suffix.
+
 ## Usage
 
 Generate LaTeX:
@@ -175,11 +181,18 @@ Short options are also available:
 ./build/kindle2notes -i input.txt -o notes.md -t md
 ```
 
+Show the version compiled into the executable:
+
+```bash
+./build/kindle2notes --version
+```
+
 | Argument | Short form | Description |
 | --- | --- | --- |
 | `--input` | `-i` | Path to the Kindle clippings file |
 | `--output` | `-o` | Path to the generated file |
 | `--type` | `-t` | Output format: `latex`, `md`, `org`, or `adoc` |
+| `--version` | `-v` | Show the compiled version and exit |
 | `--help` | `-h` | Show usage information |
 
 If an unsupported output type is requested, the program prints the list of