dep: Add vixl (AArch32/64 assembler)

dep/vixl/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"