Update README.md

[ghost]

Added Thunderbird references to match all other installation methods.

Reordered sections to provide overall document consistency. Reordered reference links and removed unneeded ones.

Renamed All Browsers section to All Programs to be more general and given the fact that Thunderbird is not a browser.

Rewrote "Do NOT" paragraph under All Programs section to provide consciousness and a more clear general message.

Combined License & Privacy Links under About section.

Other miscellaneous fixes, removals, cleanups, etc.

[ghost]

@gorhill

[gorhill]

I would send Thunderbird icon at the end, it's quite certainly the least likely installation choice someone is looking for.

[ghost]

How about now?