Improve documentation, focus on concrete instructions

https://github.com/phw/peek/issues/88#issuecomment-335297120

> But I found the start into AppImage hard. It's not that the documentation is missing something, it's more how it is structured. Both Flatpak and Snappy emphasize strongly on the tooling, so you quickly see how you can use snapcraft / flatpak-builder to get things built.
>
> AppImage's documentation focuses more on the structure and the available options. You are told to get a proper AppDir, but than you have to search what an AppDir is, how it is structured and what tools exist to create it. Likewise it tells you to gather binaries somehow, whereas Snappy / Flatpak quickly show you how to package libraries and support the common build tools out of the box. The lack of concrete instructions makes the entry rather hard.

AppImage is a volunteer-based community effort, so pull requests are highly welcome, as are [wiki](https://github.com/AppImage/AppImageKit/wiki) edits.