Add links to diffs to changelog

Author: sipa

CHANGELOG.md

@@ -29,3 +29,7 @@ However, there are unreleased versions of libsecp256k1 that are *not* ABI compat
 This version was in fact never released.
 The number was given by the build system since the introduction of autotools in Jan 2014 (ea0fe5a5bf0c04f9cc955b2966b614f5f378c6f6).
 Therefore, this version number does not uniquely identify a set of source files.
+
+[unreleased]: https://github.com/bitcoin-core/secp256k1/compare/v0.2.0...HEAD
+[0.2.0]: https://github.com/bitcoin-core/secp256k1/compare/423b6d19d373f1224fd671a982584d7e7900bc93..v0.2.0
+[0.1.0]: https://github.com/bitcoin-core/secp256k1/commit/423b6d19d373f1224fd671a982584d7e7900bc93