chore: added versions file (#218)

VERSIONS.md

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+# Versioning Strategy
+
+`twilio-go` uses a modified version of [Semantic Versioning][semver] for
+all changes to the helper library. It is strongly encouraged that you pin at
+least the major version and potentially the minor version to avoid pulling in
+breaking changes.
+
+Semantic Versions take the form of `MAJOR.MINOR.PATCH`
+
+When bugs are fixed in the library in a backwards-compatible way, the `PATCH`
+level will be incremented by one. When new features are added to the library
+in a backwards-compatible way, the `PATCH` level will be incremented by one.
+`PATCH` changes should _not_ break your code and are generally safe for upgrade.
+
+When a new large feature set comes online or a small breaking change is
+introduced, the `MINOR` version will be incremented by one and the `PATCH`
+version reset to zero. `MINOR` changes _may_ require some amount of manual code
+change for upgrade. These backwards-incompatible changes will generally be
+limited to a small number of function signature changes.
+
+The `MAJOR` version is used to indicate the family of technology represented by
+the helper library. Breaking changes that require extensive reworking of code
+will cause the `MAJOR` version to be incremented by one, and the `MINOR` and
+`PATCH` versions will be reset to zero. Twilio understands that this can be very
+disruptive, so we will only introduce this type of breaking change when
+absolutely necessary. New `MAJOR` versions will be communicated in advance with
+`Release Candidates` and a schedule.
+
+## Supported Versions
+
+Only the current `MAJOR` version of `twilio-go` is supported. New
+features, functionality, bug fixes, and security updates will only be added to
+the current `MAJOR` version.
+
+[semver]: https://semver.org