add enketo docs and references

docs/development-setup.md

@@ -219,6 +219,8 @@ so ODK's #collect-code Slack channel can be a good place to get help with error
 
 ### Upgrading Enketo
 
+See [here](lib/enketo-transformer-service/README.md) for a birds-eye overview of Enketo in NEMO.
+
 Enketo uses jQuery under the hood, and it's important to keep library versions consistent so there aren't conflicts.
 To upgrade our version of Enketo:
 1. Check their [changelog](https://github.com/enketo/enketo-core/blob/master/CHANGELOG.md)

lib/enketo-transformer-service/README.md

@@ -0,0 +1,19 @@
+# Enketo transformer service for NEMO
+
+## Bird's eye overview
+
+resources:
+- [enketo-core github](https://github.com/enketo/enketo/tree/main/packages/enketo-core)
+- [enketo-transformer github](https://github.com/enketo/enketo/blob/main/packages/enketo-transformer)
+
+architecture/flow:
+- nemo talks to `lib/enketo-transformer-service/index.js` (separate node app that runs enketo-transformer) to convert our form XML into a JSON object we're able to send to enketo
+- nemo renders a `<div id="enketo">` in `enketo_form.html.erb`
+- nemo injects 2 variables into JS via a `<script>` tag:
+  - `window.ENKETO_MODEL_STR` is the form XML from enketo-transformer-service
+  - `window.ENKETO_INSTANCE_STR` is the optional response XML (when editing)
+- enketo injects itself into `document.querySelector('#enketo form')` via jQuery in `packs/enketo.js`
+- enketo loads the two variables set above and renders the form
+- user types in info and hits submit
+- enketo submits XML to nemo's existing ODK `/submission` (`responses#create`) route
+- on edit, nemo saves the edited XML to `modified_odk_xml` attachment so as to preserve the original `odk_xml` as required by some orgs