Add information how to make a release

doc/release.md

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+## How to make a release
+
+Make sure main and data are in a clean state.
+
+    export RELEASEDATE=2022-01-17
+    # Release for the main branch
+    git tag -a v$RELEASEDATE
+    # enter in editor:
+        # v$RELEASEDATE
+        #
+        # Longer release details
+    git push origin v$RELEASEDATE
+    # Prepare data directory
+    cd data
+    # Avoid the tag being part of a branch that might be squashed
+    git co --orphan data-release
+    git rm --cached -r .
+    rm -r *
+    cd ..
+    make data-update
+    # check
+    make data-status
+    cd data
+    # `git add` should already been done by make data-status
+    # git status
+    git commit -m "Generated from main <sha>"
+    git tag -a data-$RELEASEDATE
+    # enter in editor:
+        # data-$RELEASEDATE
+        #
+        # Longer release details
+    git push origin data-$RELEASEDATE
+