Add missing libappindicator dependency to readme.md

[RubenKelevra]

@lidel wrote:

in theory ipfs-desktop dependencies are the same as any other Electron app, only caveat being libappindicator for tray icon.

Originally posted by @lidel in #1890 (comment)

Looks like this dependency is missing in the readme :)

[lidel]

We do have a note here https://github.com/ipfs/ipfs-desktop/#why-am-i-missing-the-system-tray-menu-on-linux

[lidel]

Also, I meant that Electron apps not always use tray (which is pandora's box if you want to have it across all operating systems).

Context:

• Tray support on modern linux DE is severely broken (Tray Icon problems on Linux #1153 (comment))
• Then it got even worse when Chromium switched to StatusIndicator and Electron shipped that change without considering ecosystem consequences
• Then Electron reverted that decision (Tray Icon problems on Linux #1153 (comment))

🙈