Consider adding missing documentation

[tatsumoto-ren]

In https://github.com/Rapptz/jimaku/issues/10 I mention that when I'm trying to open /admin, I'm getting immediately redirected back to /. There's no docs on managing the server, so I had to try different things. I've figured a way to access /admin. Apparently, You have to edit the database and mark yourself as admin by changing the user flag from 0 to 1. I don't know if this is the correct way, or if this is the only way, but I suggest documenting it in the README.

Additionally, I suggest explaining how to manage the website because the /admin page is rather bare-bones.

• [ ] How to add, remove, users.
• [ ] How to assign someone as an editor.
• [ ] How to add, delete, edit subtitles.

[Rapptz]

The proper way to create an admin user is by doing

./server admin

This can be found using e.g. --help. Here's some sample output from cargo run -- --help:

> cargo run -- --help
     Running `target\debug\jimaku.exe --help`
usage: jimaku <command>

commands:
  run               Runs the server
  admin             Interactively creates an admin user
  scrape   [path]   Scrapes and creates a fixture file from kitsunekko
  fixtures <path>   Loads a fixture from the given path
  move     [path]   Move directory entry paths to a new location

options:
  -h, --help   Prints this help output

Adding users is just... registering on the site. Removing users is a WIP. Assigning someone as an editor is a toggle on the user page. Adding, deleting, and editing(?) subtitles is also just done on the site.

I don't really need tools to do these off-site since that's annoying to do.

[tatsumoto-ren]

This can be found using e.g. --help.

How about this hint is added to the readme?