Add features dev and support policy doc

Related etcd-io#13775

Signed-off-by: Sahdev Zala <[email protected]>

Documentation/contributor-guide/features.md

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+# Features 
+
+This document provides guidelines for adding and removing etcd features. The etcd features fall into two stages, experimental and stable. 
+
+## Experimental
+
+Any new feature should be added as an experimental feature. An experimental feature is characterized as below:
+- Associated with an issue with a clear need for such a feature.
+- Any configuration flags related to the implementation of the feature are prefixed with `experimental` e.g. `--experimental-feature-name`
+- Any variable names related to the implementation of the feature are prefixed with `Experimental` e.g. `ExperimentalFeatureName`
+- Might be buggy. Enabling the feature may not work as expected. 
+- Support for such a feature may be dropped at any time without notice.
+- Disabled by default.
+
+### Graduation to Stable
+
+It is important that experimental features don't get stuck in that stage. An experimental feature should move to the stable stage by using the criteria provided below: 
+- The feature was enabled for at least one major release or one year, whatever comes first. This time frame should be enough to find any potential bugs in the feature.
+- There are no open issues against the feature.
+- A PR is opened with needed changes in the configuration flag name, variables, help documentation, and anything related. 
+
+## Stable
+
+A stable feature is characterized as below: 
+- Supported as part of the supported releases of etcd.
+- May be enabled by default.
+- Discontinuation of support must follow the Feature Depreciation Process. 
+
+### Feature Depreciation Process
+
+As the project evolves, sometimes a stable feature may need to be deprecated and removed. Such a situation should be handled using the steps below:
+- Provide a depreciation message in the feature usage document before a planned major release for feature depreciation. e.g. `To be deprecated in <release>`.
+- Deprecate the feature in the planned major release with an appropriate message as part of the feature usage document. e.g. `Deprecated`.
+- Deprecated feature then be removed in the following major release. 
+
+Overall, it will take two major releases before a stable feature is completely removed.