Update README.md

[ghost]

• Added table of contents
• Updated update frequency with information on beta branch and link to Flathub documentation on how to add the beta branch
• General tidying

[flathubbot]

Started test build 137307

[flathubbot]

Build 137307 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/120343/org.DolphinEmu.dolphin-emu.flatpakref

[flathubbot]

Started test build 137530

[flathubbot]

Build 137530 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/120565/org.DolphinEmu.dolphin-emu.flatpakref

[flathubbot]

Started test build 137533

[flathubbot]

Build 137533 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/120568/org.DolphinEmu.dolphin-emu.flatpakref

[flathubbot]

Started test build 137536

[flathubbot]

Build 137536 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/120571/org.DolphinEmu.dolphin-emu.flatpakref

[cpba]

Thank you!