Add developer docs

docs/dev-frontend.md

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+## Tips for frontend development
+
+The frontend assets including the Svelte js files are per default embedded in
+the bgo binary. To enable a quick turnaround cycle for web development of the
+frontend disable embedding of static assets in `config.json`:
+```
+"embed-static-files": false,
+"static-files": "./web/frontend/public/",
+
+```
+
+Start the node build process (in directory `./web/frontend`) in development mode:
+```
+$ npm run dev
+```
+
+This will start the build process in listen mode. Whenever you change a source
+files the depending javascript targets will be automatically rebuild.
+In case the javascript files are minified you may need to set the production
+flag by hand to false in `./web/frontend/rollup.config.mjs`:
+```
+const production = false
+```
+
+Usually this should work automatically.
+
+Because the files are still served by ./cc-backend you have to reload the view
+explicitly in your browser.
+
+A common setup is to have three terminals open:
+* One running cc-backend (working directory repository root): `./cc-backend -server -dev`
+* Another running npm in developer mode (working directory `./web/frontend`): `npm run dev`
+* And the last one editing the frontend source files