Adding `versioning.rst` file from the skywater-pdk.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>

docs/versioning.rst

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+Versioning Information
+======================
+
+Version Number Format
+---------------------
+
+Version numbers for both the PDK and the supplied libraries are fully specified by a 3-digit version number followed by a git commit count and a git commit short hash.
+
+The 3-digit-number will be tagged in the associated git repository as ``vX.Y.Z`` and the fully specified value can be found by running |git-describe|_ tool inside the correct git repository.
+
+.. |git-describe| replace:: ``git describe``
+.. _git-describe: https://git-scm.com/docs/git-describe
+
+The version number is broken down as ``vX.Y.Z-AAA-gHHHHH``;
+
+* The letter ``v``.
+
+* ``X`` = The "Milestone Release" Number
+
+  * **0** indicates **"alpha"** level. The IP has **not** undergone full qualification. Parts of the IP **may be immature and untested**.
+
+  * **1** indicates **"beta"** level. The IP has undergone qualification testing but has **not** been hardware verified.
+
+  * **2** indicates **production** level. The IP has passed qualification testing and has been hardware verified.
+
+* ``Y`` = The "Major Release" Number
+
+* ``Z`` = The "Minor Release" Number
+
+* A single hyphen character ``-``
+
+* ``AAA`` = The `git commit count <https://git-scm.com/docs/git-describe#_examples>`_ since the version number was tagged.
+
+* A single hyphen character followed by the letter g ``-g``
+
+* ``HHHH`` = A `git commit short hash <https://git-scm.com/book/en/v2/Git-Tools-Revision-Selection#_short_sha_1>`_ which uniquely identifies a specific git commit inside the associated git repository.