Add documentation for run_northstar.txt

[GeckoEidechse]

When doing some work on Launcher I came across this commit:
R2Northstar/NorthstarLauncher@d2ee389

which comes from this PR
R2Northstar/NorthstarLauncher#19

but was never properly documented.

I'm not sure if it's even still in use.

[GeckoEidechse]

@p0358 the mentioned PR is yours, maybe you could give some more info <3

[p0358]

It was meant to be an alternative to -northstar launch option in case someone cannot set the launch options for whatever reason or doesn't want to. The existence of this file implies you intend to run Northstar, unless the contents of this file are (starting with) 0. If the file has 0 in it that means you do not want to run Northstar on the other hand, which is equivalent to -vanilla option in NorthstarLauncher.exe, which is the opposite to -northstar in Titanfall2.exe, which means the Northstar will not be loaded even if you use NorthstarLauncher.exe.

In short:

1. run_northstar.txt exists, any content (for example 1) = Northstar is always loaded
2. run_northstar.txt exists with content 0 = Northstar is not loaded

With:

1. being mostly relevant for Titanfall2.exe (equivalent of -northstar)
2. being mostly relevant for NorthstarLauncher.exe (equivalent of -vanilla)

[p0358]

It wasn't in the end documented or widely recommended as people turned out to generally have enough brain cells to add the launch options

[GeckoEidechse]

Not sure if still wanted given that we want to get rid of the file as per R2Northstar/NorthstarLauncher#508