Added README

README.md

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+# secret
+
+Package secret provides a simple interface for fetching secrets from a secrets provider.
+
+The basic interface is:
+
+```go
+type Provider interface {
+	GetSecret(key string) (value []byte, err error)
+}
+```
+
+## Providers
+
+This package is targeting the following secrets providers:
+  * [x] Environment variables
+  * [x] Environment variables files
+  * [x] AWS [Key Management Service (KMS)](https://aws.amazon.com/kms/)
+  * [x] AWS [Session Manager Parameter Store](https://docs.aws.amazon.com/systems-manager/latest/userguide/systems-manager-parameter-store.html)
+  * [ ] Azure [Key Vault](https://azure.microsoft.com/nl-nl/products/key-vault)
+  * [ ] GCP [Secret Manager](https://cloud.google.com/security/products/secret-manager)
+  * [x] Hashicorp [Vault ](https://www.hashicorp.com/en/products/vault)
+  * [x] Keyring: Linux
+  * [x] Keyring: macOS [Keychain Access](https://support.apple.com/en-gb/guide/keychain-access/kyca1083/mac)
+  * [x] Keyring: Windows [SecretStore](https://learn.microsoft.com/en-us/powershell/utility-modules/secretmanagement/get-started/using-secretstore)
+
+## Decryption
+
+The secret provider may return encrypted values, which can be transparently descrypted
+by this package.
+
+Supported decryption methods:
+  * [x] `AES-GCM` (`AES-128-GCM`, `AES-256-GCM`) Authenticated Encryption with Associated Data
+  * [x] `RSA-OAEP` Optimal Asymmetric Encryption Padding ([RFC8017](https://www.rfc-editor.org/rfc/rfc8017.html))
+  * [x] `RSA-PKCS#1` version 1.5 ([RFC2313](https://www.rfc-editor.org/rfc/rfc2313.html))
+  * [x] `ChaCha20-Poly1305` and `XChaCha20-Poly1305` Authenticated Encryption with Associated Data ([RFC8439](https://datatracker.ietf.org/doc/html/rfc8439))
+  * [x] `NaCL Secretbox` Secret-key authenticated encryption