#79 Add a list and short summary of all commands

Currently the hardest part about knowing fpbot is that it's really hard to keep track of all the commands. I often find myself thinking, "I know there's a command to do X, but I'm not sure what it is or how to find that out."

For example, repo+ and spec+ seem like they take names of repos, not urls, so there must be a command to associate a url with a name, but I'm not sure what that command is... (and, I only found out about those commands by looking through the source code -- not something we should expect most users to do).

There is a "cheatsheet" in the README of this repo. Does it solve the problem? :)

Looks like I created this issue after the last edit to the README so no, it does not :P

Using the specific example of repo+ that drove me to create this issue, it shows the general form but does not tell me what other options are available for <repo> aside from notabug in the example (and in fact, I've forgotten it again now. Was it gitlab or snowdrift or sd to add the snowdrift repo?).

I suppose I should split this into two issues, the other one being: I'd strongly prefer if this info was available from the bot itself, with !help or !info. When I hit a snag, my first instinct is to ask the bot for guidance, not go searching for the repo's readme.

I have more specific suggestions but not the time to recount them right now. I'll be back in a few days when finals are over.