docs(versioning): add initial

Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>

Author: casaroli

docs/Versioning.md

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+# Ocre Versioning
+
+## Source components
+
+There are several components of the Ocre version that gets compiled in the project.
+
+The file `src/ocre/version.h` includes the Ocre Library version string.
+While the file `commit_id.h` includes the commit ID of the Ocre Library.
+If this file is not present in the source tree, the file is generated during the build process and is stored in the build directory.
+If the file does not exist, and the source tree is not a valid git repository, the build will fail.
+
+Note that these are unrelated to the Zephyr version and the Zephyr application version mechanism (i.e. through the use of the VERSION file in the Zephyr application directory).
+
+## Version variables
+
+These files together form the Ocre version variables, which are compiled statically into the `struct ocre_config ocre_build_configuration` struct defined in `ocre/library.h`:
+
+```c
+struct ocre_config {
+	const char *version;	/**< Version of the Ocre Library */
+	const char *commit_id;	/**< Commit ID of the build tree */
+	const char *build_info; /**< Host build information */
+	const char *build_date; /**< Build date */
+};
+
+extern const struct ocre_config ocre_build_configuration;
+```
+
+The specific meaning of these variables is as follows:
+
+### version
+
+The version of the Ocre Library. In the form Major.Minor.Patch and is defined in the `version.h` file.
+
+It follows the semantic versioning format.
+
+Note the `version.h` file is not generated automatically. Instead it is maintained manually or automatically whenever we release a new version of the Ocre Runtime.
+
+### commit_id
+
+The commit ID of the build tree.
+Generated every time into `commit_id.h` if the source tree is a valid git repository, otherwise, set to whatever is defined in `commit_id.h`.
+
+The format is `commit-hash[-dirty]`. This is generated by the build system using `git describe --always --abbrev=0 --dirty`, so it means the current commit hash and if there are any uncommitted changes in the working tree (as specificied by the optional `-dirty` suffix).
+
+### build_info
+
+The host build information. Generated on every build system by the build system automatically.
+
+It has the format `user @ system`. Where `user` is the value of the `USER` environment variable and `system` is the output of `uname -a`.
+
+### build_date
+
+The build date. Generated on every build system by the build system automatically.
+
+It has the format `YYYY-MM-DD HH:MM:SS TZ` and is generated by `date +'%Y-%m-%d %H:%M:%S %Z'`
+
+For more information about the build system, see [Linux Build System](BuildSystemLinux.md) and [Zephyr Build System](BuildSystemZephyr.md) documentation.