Add script to generate simple changelogs (#668)

scripts/changelog/.gitignore

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+changelog.md

scripts/changelog/README.md

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+# Changelog
+
+Currently, the changelog is built locally.
+Run:
+
+```
+./changelog.sh <ref_since>
+```
+
+For instance:
+```
+./changelog.sh statemine_v4
+```

scripts/changelog/changelog.sh

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+#!/usr/bin/env bash
+
+REF1=$1
+
+JSON=$(git log $REF1..HEAD \
+    --pretty=format:'{ "commit": "%H", "short_sha": "%h", "author": "%an", "date": "%ad", "message": "%s"},' \
+    $@ | \
+    perl -pe 'BEGIN{print "{ \"since\": \"'${REF1}'\",  \"commits\": ["}; END{print "]}"}' | \
+    perl -pe 's/},]/}]/')
+
+echo $JSON | tera --template templates/changelog.md --stdin | tee