Add API to add contexts to detached signatures.

Using the notation data packets of signatures, we add a way to
set a context to detached signatures.
We also add a way to enforce that signatures have the right context
during verification.

CHANGELOG.md

@@ -5,13 +5,23 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## Unreleased
-# Changed
+
+### Added
+- API for adding context to detached signatures:
+	```go
+	sig, err := keyRing.SignDetachedWithContext(message, context)
+	```
+- API to verify the context of detached signatures:
+	```go
+	err := keyRing.VerifyDetachedWithContext(message, signature, verifyTime, verificationContext)
+	```
+### Changed
 - Update `github.com/ProtonMail/go-crypto` to the latest version
 - More strictly verify detached signatures: reject detached signatures from revoked and expired keys.
 - In `GetVerifiedSignatureTimestamp`, use the new `VerifyDetachedSignatureAndHash` function to get the verified signature, instead of parsing the signature packets manually to get the timestamp.
 
 ## [2.5.2] 2022-01-25
-# Changed
+### Changed
 - Update `github.com/ProtonMail/go-crypto` to the latest version
 
 ## [2.5.1] 2022-01-24