New doc explaining vcs_version. (pantsbuild#16092)

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docs/markdown/Using Pants/generating-version-tags.md

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+---
+title: "Generating version tags from Git"
+slug: "generating-version-tags"
+hidden: false
+---
+
+Pants can generate a version string based on Git state that you can use during builds.
+
+Enabling the Python backend
+---------------------------
+
+The implementation of this feature relies on a third-party [Python library](https://github.com/pypa/setuptools_scm),
+so to use it you will need to activate the Python backend and its "experimental" extension, even if
+you're not otherwise using Python in your repo:
+
+```toml pants.toml
+backend_packages.add = [
+  ...
+  "pants.backend.python",
+  "pants.backend.experimental.python",
+  ...
+]
+```
+
+The `vcs_version` target
+------------------------
+
+To utilize this functionality, you first create a `vcs_version` target in some BUILD file, e.g.,:
+
+```python src/foo/BUILD
+vcs_version(
+    name="version",
+    generate_to="src/foo/version.py",
+    template='version = {version}',
+)
+```
+
+When you test, run or package any code that depends (directly or indirectly) on this target,
+Pants will compute a version string based on the current Git state, and then generate content
+using the specified `template` (with `{version}` serving as a placeholder for the version string)
+at the path specified by `generate_to`.
+
+This content will be available at test and run time, and be packaged along with your
+code and its dependencies. Your code can use the generated content (e.g., by importing
+it or reading it) and thus have access to a dynamically-generated version.
+
+Note that, similarly to how Pants handles other code generation tools (such as [Protobuf](doc:protobuf-python)),
+this content is not written out as a file into your source tree. Instead, the content is materialized
+on the fly, as needed.  If you want to inspect the generated contents manually, you can use the
+`export-codegen` goal:
+
+```shell
+./pants export-codegen src/foo:version
+```
+
+Using the generated version
+---------------------------
+
+Code that depends on the `vcs_version` target can import and use the generated file:
+
+```python src/util.py
+from version import version
+...
+```
+
+In this example we generated a Python source file, but you can generate code in any language,
+or even generate a text file for some code to load at runtime.
+
+In fact, in the Python case you don't even need an explicit dependency on the `vcs_version` target, as
+one will be inferred from the import. This does not work (yet) for other languages, so those will
+require an explicit dependency.
+
+The generated version string
+----------------------------
+
+Pants delegates the version string computation to [setuptools_scm](https://github.com/pypa/setuptools_scm).
+See [here](https://github.com/pypa/setuptools_scm#default-versioning-scheme) for how it computes the version string from the Git state.
+
+We don't yet support any of the configuration options that control how the string is computed. Please
+[let us know](doc:getting-help) if you need such advanced functionality.