update the frontend dependency guide with distinction between dependencies and devDependencies

doc/development/new_fe_guide/dependencies.md

 # Dependencies
 
+## Adding Dependencies.
+
+GitLab uses `yarn` to manage dependencies. These dependencies are defined in
+two groups within `package.json`, `dependencies` and `devDependencies`. For
+our purposes, we consider anything that is required to compile our production
+assets a "production" dependency. That is, anything required to run the
+`webpack` script with `NODE_ENV=production`. Tools like `eslint`, `karma`, and
+various plugins and tools used in development are considered `devDependencies`.
+This distinction is used by omnibus to determine which dependencies it requires
+when building GitLab.
+
+Exceptions are made for some tools that we require in the
+`gitlab:assets:compile` CI job such as `webpack-bundle-analyzer` to analyze our
+production assets post-compile.
+
+---
+
 > TODO: Add Dependencies