Import the VERSIONS.md file to master.

This file was attached to the 3.0.0 release, but it should live on master.
Release commits from now on will only need to update the version number in the
README.

Change-Id: I3820708586187ff8e8272985cf2ca2b3f9d2677c

VERSIONS.md

@@ -0,0 +1,30 @@
+Versioning
+==========
+
+Since version 3.0.0, VIXL uses [Semantic Versioning 2.0.0][semver].
+
+Briefly:
+
+- Backwards-incompatible changes update the _major_ version.
+- New features update the _minor_ version.
+- Bug fixes update the _patch_ version.
+
+Why 3.0.0?
+----------
+
+VIXL was originally released as 1.x using snapshot releases. When we moved VIXL
+into Linaro, we started working directly on `master` and stopped tagging
+named releases. However, we informally called this "VIXL 2", so we are skipping
+2.0.0 to avoid potential confusion.
+
+Using `master`
+--------------
+
+Users who want to take the latest development version of VIXL can still take
+commits from `master`. Our day-to-day development process hasn't changed and
+these commits should still pass their own tests. However, note that commits not
+explicitly tagged with a given version should be considered to be unversioned,
+with no backwards-compatibility guarantees.
+
+[semver]: https://semver.org/spec/v2.0.0.html
+          "Semantic Versioning 2.0.0 Specification"