Clean up the README

[rcurtin]

I've made some basic changes here to the README that hopefully highlight what this repository is now for, and also there are a number of comments inline that point out the things that still need to be addressed.

[mlpack-bot[bot]]

Thanks for opening your first pull request in this repository! Someone will review it when they have a chance. In the mean time, please be sure that you've handled the following things, to make the review process quicker and easier:

• All code should follow the style guide
• Documentation added for any new functionality
• Tests added for any new functionality
• Tests that are added follow the testing guide
• Headers and license information added to the top of any new code files
• HISTORY.md updated if the changes are big or user-facing
• All CI checks should be passing

Thank you again for your contributions! :+1:

[birm]

Have you considered using markdown comments

 [//]: # "such as this"

for some of the sections?

[rcurtin]

That's true, but I wanted the comments to be visible so that we didn't forget about them since they don't display on the repo main page. :smile: Also it gives us good motivation to resolve them relatively quickly! :)