Update README.md

[Asurasim]

Haven't done a pull request before. Let me know if I wrote too much here.

Added installation heading
Added 32 bit AUR
Added flatpak install commands
Reworded flatpak notes

• It wasn't originally clear to me what I would need to install if I were using native OBS and Flatpak games

Added note about system wide installation

• Probably unneeded however I think it will add clarification to people who are newer to Linux and are trying this. (Like me)

Reworded usage section

• Reordered them to put the recommended method first.
• Added 3rd usage method as I had issues with OpenGL on flatpak not working properly when using obs-gamecapture due to a library not being found.
• Added note about how I wrote the commands just in case somebody tries to copy and paste and doesn't replace <APP>

Added to troubleshooting

• This was from my own experience with bsnes (flatpak) and Garry's Mod (native), bsnes would crash when trying to capture and Garry's Mod wouldn't capture at all. Both of these were fixed with this.

If you feel like I have edited too much or my wording isn't great, please do change it.