Updates to README for ProtonVPN.

[jonpspri]

Breaking change

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Proposed change

Updates to the README.md for ProtonVPN to improve the ease of setup for
new adopters.
1.  Remove natpmpc caveats as natpmpc is now a part of the regular distros.
1.  Eliminate unusid PUID and PGID environment variables.
3.  Annotate private network specification to note particulars around 
    VM-hosted containers (e.g. on MacOS or Windows).

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New provider (thank you!)
• Updated provider (thank you!)
• New feature (which adds functionality to a provider script/repo usage)
• Breaking change (fix/feature causing existing functionality to break)

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue: relates to #
• Link to documentation updated (if done separately): https://...

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated