Subnautica: add Linux note to setup guide

[JoshuaEagles]

Please format your title with what portion of the project this pull request is
targeting and what it's changing.

ex. "MyGame4: implement new game" or "Docs: add new guide for customizing MyGame3"

What is this fixing or adding?

Adds a small one-line note about Linux setup for Subnautica to the setup guide, based on a conversation I (Entiss#4057) had with ReiikzThe8HeadedGooDragon™#4057 in a #tech-support thread on the Discord.

How was this tested?

Webhost ran locally to demonstrate the change made to the Markdown. ReiikzThe8HeadedGooDragon™#4057 verified the game didn't work without the launch argument change implemented and that it did work with the change implemented (and it's also something a ton of other games need for reading dll overrides for modloaders).

If this makes graphical changes, please attach screenshots.

[Berserker66]

Is Steam the only way to get Subnautica on Linux? If so this is fine, otherwise it might be good to note a generic way (such as terminal)

[JoshuaEagles]

Well, even if you didn't buy it on Steam I was picturing adding it to Steam to use Proton, but it's true that you could use other methods to run it. I added an extra sentence to account for other stores.