Documented the auth server

auth/README.md

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+# Authentication server
+
+This server handles user authentication. In our very simple case, it will only ask for an username, but we can think of improving it to interact with a LDAP server. Once the user authenticated itself, it server will place a Macaroon in the user's browser, with a status caveat, depending on the route used:
+
+* If the user authed on `/`, they will have the "student" status.
+* If the user authed on `/teacher`, they will have the "teacher" status.
+
+## Install & run
+
+Just run `npm install` in this repository to install the dependencies, then run either `npm start` or `node server.js` to start the server.