📝 Add setup documentation

.env.example

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+TWITCH_CLIENT_ID=
+TWITCH_CLIENT_SECRET=

README.md

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+# Readme
+
+Hello and thanks for choosing this tool for your streams!
+
+## Setting up
+
+First you have to create the application, go to
+[Twitch developer console](https://dev.twitch.tv/console) and create it. Once done
+make a copy of the `.env.example` named `.env` and fill the CLIENT_ID and
+CLIENT_SECRET field values provided by Twitch.
+
+You will have something similar to this:
+
+```txt
+TWITCH_CLIENT_ID=theClientIdProvidedByTwitch
+TWITCH_CLIENT_SECRET=aSecretYouHaveToKeepSafe
+```
+
+Now we have to authenticate the account we are going to use using OAuth. In order
+to get it we have to make a `GET` request to the following endpoint.
+
+```txt
+GET https://id.twitch.tv/oauth2/authorize
+  ?client_id=<your client ID>
+  &redirect_uri=<your registered redirect URI>
+  &response_type=code
+  &scope=<space-separated list of scopes>
+```
+
+You can get all the available scopes on [here](https://dev.twitch.tv/docs/authentication/#scopes).
+Don't forget to set the exact same redirect_url as you did on the application.
+
+**Important**: you have to perform this operation with the account you want to use
+to send the messages. You can have two different accounts, the owner of the
+application and the authorized account, aka "the chatter". But I recommend you
+to use the same account for simplicity.
+
+Once authorized, we will get redirected to the specified address. The url will
+have a `GET` parameter called `code` that we will use to obtain the access token.
+
+```txt
+http://localhost/?code=<code of the authorization>
+  &scope=channel:manage:redemptions channel:read:hype_train channel:read:polls channel:read:predictions channel:read:redemptions channel:moderate chat:edit chat:read
+```
+
+Finally make a `POST` request to the following url.
+
+```txt
+POST https://id.twitch.tv/oauth2/token
+  ?client_id=<your client ID>
+  &client_secret=<your client secret>
+  &code=<authorization code received above>
+  &grant_type=authorization_code
+  &redirect_uri=<your registered redirect URI>
+```
+
+This will return a JSON-encoded response. Copy the whole response into a file
+called `tokens.json` and place it on the root of this project (same place as
+is this file).
+
+The JSON has this format:
+
+```json
+{
+  "access_token": "the access token",
+  "expires_in": 14361,
+  "refresh_token": "the refresh token",
+  "scope": [
+    // list of the scopes
+  ],
+  "token_type": "bearer"
+}
+```
+
+## Starting the service
+
+Install any dependencies executing the following command `yarn`, then you can
+start the service using the start script `yarn start`. Keep the terminal open
+until you want to close the service.
+
+## Development
+
+In order to start the service in development mode use the dev script `yarn dev`,
+this will reload the backend whenever you make any change. Keep in mind that some
+of the features might be disabled in this environment.