[admin] Add contrib component rules (#382)

* Add contrib component rules

* Move unmaintained process to general rules.

* Make sure extensions are included

distributions/README.md

@@ -11,6 +11,7 @@ Distributions supported by the Collector SIG should fulfill the following criter
 3. Should not be specific to any vendor.
 4. May be focused on development or proof of concept use cases.  The distribution should clearly indicate whether the Collector SIG recommends the distribution be used in production environments.
 5. Must only include components from the `opentelemetry-collector` and `opentelemetry-collector-contrib` repositories.
+    - Components that are marked [Unmaintained](https://github.com/open-telemetry/opentelemetry-collector#unmaintained) will be kept in any distributions for six months. After six months of being unmaintained the component will be removed from the distributions.
 6. Have a clearly defined list of criteria for which components are included.
 7. Must include the following assets except where the specific purpose of the distribution is naturally associated with a subset of these assets. In such cases, it should be clearly stated which assets are skipped and why.  Additional assets may be included if the maintainers agree:
     - Binaries for linux_amd64, linux_arm64, windows_amd64 and darwin_arm64

distributions/otelcol-contrib/README.md

@@ -14,3 +14,7 @@ Building a [custom collector](https://opentelemetry.io/docs/collector/custom-col
 ## Components
 
 The full list of components is available in the [manifest](manifest.yaml)
+
+### Rules for Component Inclusion
+
+- Include all extensions at [Alpha stability](https://github.com/open-telemetry/opentelemetry-collector#alpha) or higher and pipeline components that have at least 1 signal at [Alpha stability](https://github.com/open-telemetry/opentelemetry-collector#alpha) or higher.