docs: Add versioning file (aws#224)

Author: seebees

VERSIONING.md

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+# Versioning Policy
+
+The AWS Encryption SDK for JavaScript versioning follows [semantic versioning][link-semver] standards.
+
+## Major versions
+
+Major version changes are significant and expected to break backwards compatibility.
+
+## Minor versions
+
+Minor version changes will not break compatibility between the previous minor versions;
+to do so is a bug.
+Encryption SDK changes will also involve addition of optional features, and non-breaking enhancements.
+Additionally, any change to the version of a dependency is a minor version change.
+
+## Patch versions
+
+Patch versions changes are meant only for bug fixes,
+and will not break compatibility of the current major version.
+A patch release will contain a collection of minor bug fixes, 
+or individual major and security bug fixes, depending on severity.
+
+# Semantic Commits
+
+We seek to increase clarity at all levels of the update and releases process.
+We require pull requests adhere to the [Conventional Commits][conventional-commits] spec,
+which can be summarized as follows:
+
+* Commits that would result in a semver **major** bump must start with `BREAKING CHANGE:`.
+* Commits that would result in a semver **minor** bump must start with `feat:`.
+* Commits that would result in a semver **patch** bump must start with `fix:`.
+
+* We allow squashing of commits,
+  provided that the squashed message adheres the the above message format.
+
+* It is acceptable for some commits in a pull request to not include a semantic prefix,
+  as long as a later commit in the same pull request contains a meaningful encompassing semantic message.
+
+[link-semver]:https://semver.org/
+[conventional-commits]: https://conventionalcommits.org/