SNOW-1636484 document certificate checking in .NET driver

[sfc-gh-dszmolka]

Description

This is a documentation-only change, describing how certificate validation works today in .NET driver as opposed to all the other Snowflake drivers, and describing some possible gotchas to pay attention for.

Checklist

• [ ] Code compiles correctly
• [ ] Code is formatted according to Coding Conventions
• [ ] Created tests which fail without the change (if possible)
• [ ] All tests passing (dotnet test)
• [x] Extended the README / documentation, if necessary
• [x] Provide JIRA issue id (if possible) or GitHub issue id in PR name

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.66%. Comparing base (c3cb344) to head (738741e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1015 +/- ## ========================================== - Coverage 85.70% 85.66% -0.04% ========================================== Files 117 117 Lines 11331 11331 Branches 1130 1130 ========================================== - Hits 9711 9707 -4 - Misses 1342 1345 +3 - Partials 278 279 +1 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.