Add versioning link and doc to README (#22)

README.md

@@ -33,6 +33,10 @@ Eventually, the plan is to have self-contained packages for \*nix systems and an
 1. Check if there's a new version: `npm outdated -g twilio-cli`
 1. Update the CLI globally: `npm update -g twilio-cli`
 
+## Versioning
+
+`twilio-cli` uses a modified version of [Semantic Versioning](https://semver.org) for all changes. [See this document](VERSIONS.md) for details.
+
 ## Basic usage
 
 ### Step 1 - Login (aka add a project)

VERSIONS.md

@@ -0,0 +1,34 @@
+# Versioning Strategy
+
+`twilio-cli` uses a modified version of [Semantic Versioning][semver] for
+all changes. 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 CLI in a backwards-compatible way, the `PATCH` level
+will be incremented by one. When new features are added to the CLI 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 command changes.
+
+The `MAJOR` version is used to indicate the family of technology represented by
+the CLI. 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-cli` is supported. New features,
+functionality, bug fixes, and security updates will only be added to the
+current `MAJOR` version.
+
+[semver]: https://semver.org