Add a change log.

CHANGES.md

+# Changelog
+
+## Breaking changes
+
+A series of commits ending in commit
+6d20f3a928697d5b4bcb507b60b197407dc9a70a on April 22, 2021 introduced
+breaking changes and some additions to the VMOD:
+
+- Unrecoverable errors now lead to VCL failure. Previously, functions
+  and methods returned sentinel values under error conditions (such as
+  NULL for string returns) and continued processing. This could lead
+  to fatal errors going unnoticed. VCL failure was already used for
+  the set integer() method, since there is no sentinel value for
+  integers. See the ERRORS section of README and the message in commit
+  c70e410a8a152453bd428278dca9ad9045bf4ebd for details.
+
+- Added the methods `.subroutine()` and `.check_call()`.
+
+The changed error handling is a breaking change. Typically this may
+nevertheless require few or no changes in production VCL, since these
+are "can't happen" errors that are ordinarily discovered in
+development and testing, and don't make it into production (or else
+they are noticed quickly). But uses of the new version should be
+checked nonetheless.
+
+In detail:
+
+### Changed
+
+- Unrecoverable errors now lead to VCL failure. See the documentation
+  for details, in particular the ERRORS section of the manual
+  (README).
+
+- Set `.compile()` is now unnecessary and deprecated. Set compile
+  is now done automatically when `vcl_init` completes.
+
+### Added
+
+- Set methods `.subroutine()` and `.check_call()`