explain CSRF issue related to USE_HTTPS

reasoning: people may blow past this and it's helpful to have something
they can see explaining why they got an error and how to fix it.

content/contributing/install-dev.md

@@ -18,7 +18,7 @@ cp .env.example .env
 ```
 - In `.env`, change `DEBUG` to `true`
 - Optionally, you can use a service like [ngrok](https://ngrok.com/) to set up a domain name, and set the `DOMAIN` variable in your `.env` file to the domain name generated by ngrok.
-- Unless you want to set up your local machine to support https during development, you may want to set `USE_HTTPS=false`
+- Unless you want to set up your local machine to support https during development, you may want to set `USE_HTTPS=false`. If you try to register your admin account and see a message that `CSRF verification failed`, you should set this option.
 
 - Set up nginx for development by copying the developer nginx configuration file (`nginx/development`) into a new file named `nginx/default.conf`:
 ``` { .sh }