Add versioning documentation (open-telemetry#379)

VERSIONING.md

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+# Versioning
+
+This document describes the versioning policy for this repository. This policy
+is designed so the following goals can be achieved.
+
+## Goals
+
+### API Stability
+
+Once the API for a given signal (spans, logs, metrics, baggage) has been
+officially released, that API module will function with any SDK that has the
+same major version, and equal or greater minor or patch version.
+
+For example, libraries that are instrumented with `opentelemetry 1.0.1` will
+function in applications using `opentelemetry 1.11.33` or `opentelemetry
+1.3.4`.
+
+### SDK Stability
+
+Public portions of the SDK (constructors, configuration, end-user interfaces)
+must remain backwards compatible. Internal types are allowed to break.
+
+## Policy
+
+* Releases will follow [SemVer](https://semver.org/).
+* New telemetry signals will be introduced behind experimental 
+  [cargo features](https://doc.rust-lang.org/cargo/reference/features.html).
+
+  * New signals will be stabilized via a **minor version bump**, and are not
+    allowed to break existing stable interfaces.
+
+* GitHub releases will be made for all released versions.
+* Crates will be released on crates.io
+
+## Example Versioning Lifecycle
+
+To better understand the implementation of the above policy, here is an example
+of how the metrics and logging signals **could** stabilize.
+
+- v1.0.0 release:
+   - `opentelemetry 1.0.0`
+     - Contains stable impls of trace, baggage, resource, context modules
+     - experimental metrics impl behind feature flag
+   - `opentelemetry-semantic-conventions 1.0.0`
+     - Contains stable impls of trace, resource conventions
+     - experimental metrics conventions behind feature flag
+   - `opentelemetry-contrib 1.0.0`
+     - Contains stable impls of 3rd party trace exporters and propagators
+     - experimental metrics exporters and propagator impls behind feature flag
+- v1.5.0 release (with metrics)
+   - `opentelemetry 1.5.0`
+     - Contains stable impls of metrics, trace, baggage, resource, context modules
+     - experimental logging impl still only behind feature flag
+   - `opentelemetry-semantic-conventions 1.2.0`
+     - Contains stable impls of metrics, trace, resource conventions
+     - experimental logging conventions still only behind feature flag
+   - `opentelemetry-contrib 1.6.0`
+     - Contains stable impls of 3rd party trace and metrics exporters and propagators
+     - experimental logging exporters and propagator still impls behind feature flag
+- v1.10.0 release (with logging)
+   - `opentelemetry 1.10.0`
+     - Contains stable impls of logging, metrics, trace, baggage, resource, context modules
+   - `opentelemetry-semantic-conventions 1.4.0`
+     - Contains stable impls of logging, metrics, trace, resource conventions
+   - `opentelemetry-contrib 1.12.0`
+     - Contains stable impls of 3rd party trace, metrics, and logging exporters and propagators