Move the Installation Guide to the top

[Rigerhit047]

I feel like it's a really crucial read for people new to Firefox theming to be relegated to the bottom of the page where it will be usually be ignored, when it could fit perfectly besides Themes List on the top.

Also adding in the guide that themes can ask for more permissions or make their custom ones and how to add them to the about:config page along with a warning to read (if any) the readme of the theme to install, also how to know their directory from about:support or about:profiles (Making VERY sure to point to the correct directory) if they aren't on the list of provided ones/they can't find the respective folders would be good advice for saving headaches and conflicting instructions in some themes.

[unseen-ninja]

I do agree that this is potentially incomplete. The guide provided on this repo is more of a very general guideline. As you've mentioned yourself it's not going to work for every theme as a lot of them need extra configuration. Hence it's more important to refer to the themes instructions rather than the steps listed here as doing otherwise may very well result in an influx of issues a la »Installing THEME doesn't work« being raised which would be completely out of scope here.