Restore Vision document (open-telemetry#764)

This was lost accidentally in open-telemetry#581

README.md

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   <strong>
     <a href="CONTRIBUTING.md">Contributing<a/>
     &nbsp;&nbsp;&bull;&nbsp;&nbsp;
+    <a href="docs/vision.md">Vision<a/>
+    &nbsp;&nbsp;&bull;&nbsp;&nbsp;
     <a href="docs/design.md">Design<a/>
     &nbsp;&nbsp;&bull;&nbsp;&nbsp;
     <a href="docs/monitoring.md">Monitoring<a/>

docs/vision.md

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+# OpenTelemetry Collector Long-term Vision
+
+The following are high-level items that define our long-term vision for OpenTelemetry Collector, what we aspire to achieve. This vision is our daily guidance when we design new features and make changes to the Collector.
+
+This is a living document that is expected to evolve over time.
+
+## Performant
+Highly stable and performant under varying loads. Well-behaved under extreme load, with predictable, low resource consumption.
+
+## Observable
+Expose own operational metrics in a clear way. Be an exemplar of observable service. Allow configuring the level of observability (more or less metrics, traces, logs, etc reported). See [more details](observability.md).
+
+## Multi-Data
+Support traces, metrics, logs and other relevant data types.
+
+## Usable Out of the Box
+Reasonable default configuration, supports popular protocols, runs and collects out of the box.
+
+## Extensible
+Extensible and customizable without touching the core code. Can create custom agents based on the core and extend with own components. Welcoming 3rd party contribution policy.
+
+## Unified Codebase
+One codebase for daemon (Agent) and standalone service (Collector).
+
+For more details on how we plan to achieve this vision please see the [Roadmap](roadmap.md).