Improve readability in readme, move installation section up

I find the readme more readable this way.
The installation have moved up above the file-size-section. IMO the 1,2,3,4 steps are more important than the file-sizes.

Also merged the installation step 1 with the requirements (less redundancy).

And change the markdown formatting to have nicer code-formatting. The change numberformat is required to work around the issue that #3 restarts as #1 when using the auto-numbering.