Add dev setup instructions

DEV-SETUP.md

@@ -0,0 +1,41 @@
+# Developer Setup
+To get this up and running, you need the following tools installed on your machine:
+
+- Node.js **v14 or later**
+    - https://nodejs.org/en/download/
+- The Yarn package manager
+    - https://classic.yarnpkg.com/en/docs/install/
+
+Once you've done that, open a shell to this repo and run the following:
+
+```shell script
+yarn install
+```
+
+This will install all the 3rd-party libraries needed for the app to start.
+
+Next, copy the `example.env` file to `.env` and edit the `.env` file we just created.
+
+Around line 20, modify the `DATABASE_PASSWORD=` to include the development database password, which you can find from the link shared on Discord.
+
+For example:
+
+```shell script
+DATABASE_PASSWORD=abc12345
+```
+
+Once you've done that, you should be able to start the application and it will connect to the shared database. To run the application, run:
+
+```shell script
+node index.js
+```
+
+Note that, if you make any backend changes, you'll need to restart the server in order for them to take effect.
+
+Once you've started the server, you should see a line:
+
+```
+ [SUCCESS] [Server] Flitter running on port 8000! Press ^C to exit cleanly.
+```
+
+Then, open your web browser to http://localhost:8000 to see the site.