add document defining an openTelemetry Collector

Related to open-telemetry#4309

Signed-off-by: Alex Boten <[email protected]>

specification/collector/README.md

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+<!--- Hugo front matter used to generate the website version of this page:
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+  from: tmp/otel/specification/baggage/_index.md
+  to: baggage/README.md
+--->
+
+# OpenTelemetry Collector
+
+The goal of this document is for users to be able to easily switch between
+OpenTelemetry Collector Distros while also ensuring that components produced by
+the OpenTelemetry Collector SIG is able to work with any vendor who claims
+support for an OpenTelemetry Collector.
+
+- An OpenTelemetry Collector _MUST_ accept a OpenTelemetry Collector Config file.
+- An OpenTelemetry Collector _MUST_ be able to be compiled with any and all
+  additional Collector plugins that the user wishes to include.
+- A compiled instance of an OpenTelemetry Collector – with a specific set of
+  plugins and features – is referred to as an OpenTelemetry Collector Distro.