Discord: add developer notes to the README (#1728)

This is based on notes taken during exploring of the API and building
the hackathon prototype. As the production-ready version is being built
right now, they seem like useful notes to keep on hand.

src/plugins/discord/README.md

@@ -0,0 +1,68 @@
+# SourceCred Discord plugin
+
+This plugin loads data from a Discord server.
+
+## Developer notes
+
+Discord developer docs can be found at:
+https://discordapp.com/developers/docs
+
+### Setting up a bot
+
+To query the API with a reasonable rate-limit, you should set up a Discord app,
+and a Discord bot as part of that app.
+
+```
+Discord app
+ └── Discord bot
+```
+
+You can create both on the developer portal here:
+https://discordapp.com/developers/applications
+
+
+Permissions this bot will need:
+
+- `View Channels`
+- `Read Message History`
+
+Represented as `66560` integer.
+
+Then, someone with appropriate permissions needs to invite this bot to the server.
+There isn't a simple generator for this link on the dev portal, you'll need to format it yourself like this:
+
+`https://discordapp.com/api/oauth2/authorize?client_id={{clientID}}&scope=bot&permissions={{permissionsInt}}`
+
+And open it in the browser, as a logged-in server admin.
+
+Read more: https://discordapp.com/developers/docs/topics/oauth2#bot-authorization-flow
+
+### Authenticating API requests
+
+You'll want to set your bot's token in a bot style Authorization header.
+
+https://discordapp.com/developers/docs/reference#authentication
+
+`Authorization: Bot MTk4NjIyNDgzNDcxOTI1MjQ4.Cl2FMQ.ZnCjm1XVW7vRze4b7Cq4se7kKWs`
+
+### Finding configuration values / parameters
+
+Many of the ID's you will need for configuration are exposed using the Discord client.
+
+By enabling developer mode (under Appearance > Advanced > Developer Mode).
+A right-click menu option "Copy ID" will appear.
+
+Not every element will support this "Copy ID". For example custom emoji.
+You can find some of those out by using the browser version of Discord and inspecting the DOM.
+
+Otherwise you can also use the API and query for it yourself with tools like Postman or CURL.
+
+
+With the bot invited and auth set up. Find out the Guild ID using:
+
+`GET https://discordapp.com/api/users/@me/guilds`
+
+Find your custom emoji using:
+
+`GET https://discordapp.com/api/guilds/{{discordGuildId}}/emojis`
+