documentation: Instructions for local development (#2180)

* Fix the instruction for local development

* update readme for Slack OAuth in local development

* Fix the callback URL setting instruction

README.md

@@ -96,7 +96,8 @@ For contributing features and fixes you can quickly get an environment running u
     1. `SLACK_KEY` (this is called "Client ID" in Slack admin)
     1. `SLACK_SECRET` (this is called "Client Secret" in Slack admin)
 1. Configure your Slack app's Oauth & Permissions settings 
-    1. Add `http://localhost:3000/auth/slack.callback` as an Oauth redirect URL
+    1. Slack recently prevented the use of `http` protocol for localhost. For local development, you can use a tool like [ngrok](https://ngrok.com) or a package like `mkcert`. ([How to use HTTPS for local development](https://web.dev/how-to-use-local-https/))
+    1. Add `https://my_ngrok_address/auth/slack.callback` as an Oauth redirect URL
     1. Ensure that the bot token scope contains at least `users:read`
 1. Run `make up`. This will download dependencies, build and launch a development version of Outline