Issue #1679. Update dev-env-setup.md to mention the need to re-build …

…after modifying .jst templates.

docs/dev-env-setup.md

@@ -198,6 +198,7 @@ After certain kinds of changes are made, you need to build the project before se
 
 * CSS: a build will run cssnext, combine custom media queries, and concat all source files into webcompat.dev.css. You'll need to re-build the CSS to see any changes, so it's recommended to use a watch task (see `npm run watch`).
 * JS: a build will run eslint, minify and concat source files.
+* JS templates (.jst files): if you are making changes to a Backbone template in a `.jst` file, you will need to re-run the `build` command to update the pre-compiled `templates.js` file before you will see the results.
 * HTML templates: the changes should be served from disk without the need for rebuilding
 * Python: the Flask local server will detect changes and restart automatically. No need to re-build.