Docs for SecretKit (#306)

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Sources/Packages/Sources/SecretKit/Documentation.docc/Documentation.md

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+# ````SecretKit````
+
+SecretKit is a collection of protocols describing secrets and stores.
+
+## Topics
+
+### Base Protocols
+
+- ``Secret``
+- ``SecretStore``
+- ``SecretStoreModifiable``
+
+### Store List
+
+- ``SecretStoreList``
+
+### Type Erasers
+
+- ``AnySecret``
+- ``AnySecretStore``
+- ``AnySecretStoreModifiable``
+
+### OpenSSH 
+
+- ``OpenSSHKeyWriter``
+- ``OpenSSHReader``
+
+### Signing Process
+
+- ``SignedData``
+- ``SigningRequestProvenance``