Add FEATURES.md and DEPRECATIONS.md (sigstore#1429)

Signed-off-by: Priya Wadhwa <priya@chainguard.dev>

DEPRECATIONS.md

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+# Deprecations
+
+This doc lists deprecated features in `cosign`.
+You can read more about Sigstore's deprecation policy [here](https://docs.sigstore.dev/api-stability)!
+
+| **Feature Being Deprecated** | **API Stability Level** | **Earliest Date of Removal** |
+|------------------------------|-------------------------|------------------------------|
+| My feature                   | Experimental/Beta/GA    | DD/MM/YY                     |
+|                              |                         |                              |
+|                              |                         |                              |

FEATURES.md

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+# Feature Stability
+
+This doc covers feature stability in `cosign` as described in the [API Stability Policy](https://docs.sigstore.dev/api-stability) for Sigstore.
+
+## Experimental
+* Keyless signing using the `Fulcio` CA
+* Storing signatures in a transparency log
+* The `pkg/cosign/oci` client library
+
+Some formats that cosign relies upon are not stable yet either:
+* The SBOM specification for storing SBOMs in a container registry
+* The In-Toto attestation format
+
+
+## Beta
+* All cosign subcommands, including flags and output
+
+
+## General Availability
+
+### Key Management
+
+* fixed, text-based keys generated using `cosign generate-key-pair`
+* cloud KMS-based keys generated using `cosign generate-key-pair -kms`
+* keys generated on hardware tokens using the PIV interface using `cosign piv-tool`
+* Kubernetes-secret based keys generated using `cosign generate-key-pair k8s://namespace/secretName`
+
+
+### Artifact Types
+
+* OCI and Docker Images
+* Other artifacts that can be stored in a container registry, including:
+  * Tekton Bundles
+  * Helm Charts
+  * WASM modules
+* Text files and other binary blobs, using `cosign sign-blob`