Add some development notes.

packages/tychon/DEVELOPMENT.md

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+# Development notes
+
+All copies of some files are identical. This should be maintained.
+
+It is especially important for the `common_init.yml` and `common_host.yml`
+pipeline definitions that are shared across many data streams.
+
+To check whether there are multiple versions of each of these files, use the
+following commands to list them with their MD5 hashes.
+
+```
+echo; find -name 'common_host.yml' | xargs md5sum | sort
+echo; find -name 'common_init.yml' | xargs md5sum | sort
+echo; find -name 'default_policy.json' | xargs md5sum | sort
+echo; find -name 'docker-compose.yml' | xargs md5sum | sort
+echo; find -name 'test-default-config.yml' | xargs md5sum | sort
+echo; find -wholename './elasticsearch/transform/*/manifest.yml' | xargs md5sum | sort
+```
+
+It is also important to keep the field definitions for each transform
+destination index aligned with the field definitions for its source data
+stream. (Although they will differ in `is-transform-source-*.yml` files, and
+the source data streams do not need to explicitly define ECS fields.)
+
+There are other files that have only minor differences between data streams.
+When making a change to one data stream, consider whether there is a
+corresponding change to make in other data streams.