Better docs!

[spenserblack]

#212 Was closed once the docs were fixed (no broken tags, no blatantly false info), but the docs still have room for improvement. If you like writing documentation, please submit a PR, and feel free to ask me about anything that seems confusing!

IMO, the following subjects can be improved:

• What is repofetch?
• How do I write a plugin?
  • This is in CONTRIBUTING.md, but maybe it should be documented somewhere else
  • There should be more guidelines, something like below:
    • "Do not abuse environment variables. They're for values that rarely change, like secrets. They are not a substitute for CLI arguments. Also, consider using the config file instead of the dotenv file. (A clear difference between when to use environment variables, CLI args, and config options should be defined)
    • How to create ASCII art?
    • How to define my own color themes?
    • What exceptions should I raise?