Update Changelog to mark the current status as release 0.2

spec/changelog.tex

-\subsection*{Unreleased}
+\subsection*{0.2 --- \DTMdisplaydate{2022}{12}{19}{-1}}
 
-Apply major changes to the structure of the document to clarify the difference
-between the \emph{language} and the \emph{rules}.  The language has a formal
-definition and a proof of soundness.  The syntax describes how proofs are
-encoded in the text file.
+This is an intermediate release.  It collects all changes to the original
+specification document before the major changes that were implemented as
+part of Hans-Jörg Schurr's PhD thesis.   These changes will be reflected
+in release 0.3.
+
+This release implements major changes to the structure of the document to
+clarify the difference between the \emph{language} and the \emph{rules}.
+The language has a formal definition and a proof of soundness.  The syntax
+describes how proofs are encoded in the text file.
 
 The syntax was extended to allow extra annotations.  Tools consuming Alethe
 proofs must be able to ignore such extra annotations.