Add Usage Analyzers documentation to README

[Copilot]

Documents the optional usage analyzers (NEEG004-NEEG011) that suggest replacing System.Enum methods with generated alternatives for better performance.

Changes

• Enabling analyzers: Add .globalconfig with netescapades.enumgenerators.usage_analyzers.enable = true
• Configuring severity: Use .editorconfig to set individual analyzer severities:

  [*.{cs,vb}]
  # NEEG004: Use ToStringFast() instead of ToString()
  dotnet_diagnostic.NEEG004.severity = warning
  # NEEG005: Use HasFlagFast() instead of HasFlag()
  dotnet_diagnostic.NEEG005.severity = warning
  # ... (NEEG006-NEEG011 for IsDefined, Parse, GetNames, GetValues, GetValuesAsUnderlyingType, TryParse)

• Code fixes: Notes that automatic fixes are available for all analyzers

Section placed after "Enabling interception" and before "Embedding the attributes in your project".

Original prompt

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This section details on the original issue you should resolve

<issue_title>Update READMe about Usage Analyzers</issue_title>
<issue_description>We have added some analyzers that help to encourage the use of the generated methods (found in the src/NetEscapades.EnumGenerators/Diagnostics/UsageAnalyzers folder).

• Add an additional section to the README describing these analyzers
• Be brief, giving only the information required to get started.
• We should explain how to enable the analyzers, by adding a .globalconfig file (as can be found in tests/NetEscapades.EnumGenerators.IntegrationTests/.globalconfig)
• We should explain how to (optionally) change the severity (after enabling the analyzers), using an .editorconfig file (as can be found in tests/NetEscapades.EnumGenerators.IntegrationTests/.editorconfig)
• Explain that code fixes are available (I'll add some screenshots later)</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Update README about Usage Analyzers #215

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[andrewlock]

LGTM