Updates to ensure custom servers.json files merge properly

[justinhorner]

• added context to the custom servers.json section to ensure future users better understand what to update, so that custom servers.json files will not be overwritten by the default file
  • see Bug: Manually curated server list bound to /gluetun/servers.json gets overwritten gluetun#2357 (comment) for more context / information about the 'bug'

[qdm12]

First of all thanks for this.

however my view on it is that this should be purely automated. The point of the bind mounted servers.json is so that you can automatically update it and gluetun will use it (merged together with its own built-in data). In case I vanish, gluetun should still work using the updating mechanism in place for example. The idea was never that the user should modify it manually, and I still think that's a bad idea - better to fix/PR-to-fix the updating code instead so it benefits everyone.

Now, this as it is won't be merged; however what I just described might not be obvious and, in that case, please change the documentation to your liking so everyone understands this should not be modified manually.

Why did you modify it manually? 👀