mention TOML_DISABLE_CONDITIONAL_NOEXCEPT_LAMBDA in README.md

[N-Dekker]

What does this change do?

It adds TOML_DISABLE_CONDITIONAL_NOEXCEPT_LAMBDA to the table of configurable options, in README.md.

Is it related to an existing bug report or feature request?

• It's a follow-up to pull request add TOML_DISABLE_CONDITIONAL_NOEXCEPT_LAMBDA workaround MSVC error C2057 #247

Pre-merge checklist

• I've read CONTRIBUTING.md
• I've rebased my changes against the current HEAD of origin/master (if necessary)
• I've added new test cases to verify my change
• I've regenerated toml.hpp (how-to)
• I've updated any affected documentation
• I've rebuilt and run the tests with at least one of:
  • Clang 8 or higher
  • GCC 8 or higher
  • MSVC 19.20 (Visual Studio 2019) or higher
• I've added my name to the list of contributors in README.md

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[marzer]

Thanks :)