Updates documentation :)

docs/developing.md

@@ -47,6 +47,17 @@ Note:
 - If you are using NPM, replace `yarn` with `npm run`
 - If you are using Docker, precede each command with `docker exec -it [container-id]`. Container ID can be found by running `docker ps`
 
+### Environmental Variables
+
+- `PORT` - The port in which the application will run (defaults to `4000` for the Node.js server, and `80` within the Docker container)
+- `VUE_APP_DOMAIN` - The URL where Dashy is going to be accessible from. This should include the protocol, hostname and (if not 80 or 443), then the port too, e.g. `https://localhost:3000`, `http://192.168.1.2:4002` or `https://dashy.mydomain.com`
+
+All environmental variables are optional. Currently there are not many environmental variables used, as most of the user preferences are stored under `appConfig` in the `conf.yml` file.
+
+If you do add new variables, ensure that there is always a fallback (define it in [`defaults.js`](https://github.com/Lissy93/dashy/blob/master/src/utils/defaults.js)), so as to not cause breaking changes. Don't commit your `.env` file to git, but instead take a few moments to document what you've added under the appropriate section. Try and follow the concepts outlined in the [12 factor app](https://12factor.net/config), as these are good practices.
+
+Any environmental variables used by the frontend are preceded with `VUE_APP_`. Vue will merge the contents of your `.env` file into the app in a similar way to the ['dotenv'](https://github.com/motdotla/dotenv) package, where any variables that you set on your system will always take preference over the contents of any `.env` file.
+
 ### Resources for Beginners
 New to Web Development? Glad you're here! Dashy is a pretty simple app, so it should make a good candidate for your first PR. Presuming that you already have a basic knowledge of JavaScript, the following articles should point you in the right direction for getting up to speed with the technologies used in this project:
 - [Introduction to Vue.js](https://v3.vuejs.org/guide/introduction.html)
@@ -79,7 +90,6 @@ The most significant things to note are:
 
 For the full styleguide, see: [github.com/airbnb/javascript](https://github.com/airbnb/javascript) 
 
-
 ### Frontend Components
 
 All frontend code is located in the `./src` directory, which is split into 5 sub-folders:
@@ -122,6 +132,9 @@ Running `yarn upgrade` will updated all dependencies based on the ranges specifi
 #### Performance - Lighthouse
 The easiest method of checking performance is to use Chromium's build in auditing tool, Lighthouse. To run the test, open Developer Tools (usually F12) --> Lighthouse and click on the 'Generate Report' button at the bottom.
 
+#### Dependencies - BundlePhobia
+[BundlePhobia](https://bundlephobia.com/) is a really useful app that lets you analyze the cost of adding any particular dependency to an application
+
 ### Directory Structure
 
 #### Files in the Root: `./`