docs: Update README.md

[turicfr]

• Mention the Android application of Ruffle and link to its build instructions
• Add missing folders in the sructure section
• Misc changes

[Dinnerbone]

Strictly speaking, some of these are general linux prerequisites - not specifically desktop.
I know that's not new to this PR though :D

[sleepycatcoding]

Might be worth it to add a note that these are Ubuntu packages and that the names might not match with other flavors of Linux (e.g. Fedora), just for example #14654 (comment).

[turicfr]

I'm not a Linux expert, but I know that at least Arch depends on alsa-lib, openssl, zlib, and libxcb. Might worth adding these as well?

Also, we might also want to delete default-jre-headless from the list as Java is already present in the general prerequisites.

[sleepycatcoding]

I'm not a Linux expert, but I know that at least Arch depends on alsa-lib, openssl, zlib, and libxcb. Might worth adding these as well?

libasound2 should be equivalent to alsa-lib, I think it might be worth it to have separate lists for different flavors.

Also, we might also want to delete default-jre-headless from the list as Java is already present in the general prerequisites.

I think we can keep it in, no harm mentioning it twice, just in case the reader missed it the first time.

[Dinnerbone]

Teeny tiny nit but otherwise thank you so much!

[Dinnerbone]

Thank you!