Add VERSIONING.rst. (aws#84)

VERSIONING.rst

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+*****************
+Versioning Policy
+*****************
+
+We use a three-part X.Y.Z (Major.Minor.Patch) versioning definition, as follows:
+
+* **X (Major)** version changes are significant and expected to break backwards compatibility.
+* **Y (Minor)** version changes are moderate changes. These include:
+
+  * Significant non-breaking feature additions.
+  * Any change to the version of a dependency.
+  * Possible backwards-incompatible changes. These changes will be noted and explained in detail in the release notes.
+
+* **Z (Patch)** version changes are small changes. These changes will not break backwards compatibility.
+
+  * Z releases will also include warning of upcoming breaking changes, whenever possible.
+
+What this means for you
+=======================
+
+We recommend running the most recent version. Here are our suggestions for managing updates:
+
+* X changes will require some effort to incorporate.
+* Y changes will not require significant effort to incorporate.
+
+  * If you have good unit and integration tests, these changes are generally safe to pick up automatically.
+
+* Z changes will not require any changes to your code. Z changes are intended to be picked up automatically.
+
+  * Good unit and integration tests are always recommended.