Tidy README and link StrEnum.Npgsql for Postgres enum mapping

[dmytro-khmara]

Summary

• Adds a callout in the Migrations section pointing Postgres users to the new StrEnum.Npgsql package for native Postgres enum type mapping.
• Light readability pass: Allows to use → Lets you use, missing trailing period on the EF Core support line, "mark the property is non-required" → "as non-required", trims a couple of conversational lead-ins.
• Adds ### Defining a string enum and an entity and ### Wiring it up headings so the first usage block isn't headless.
• No code changes; only README copy.

Test plan

• Render the README and verify formatting / the new blockquote callout
• Confirm all sample code blocks still parse cleanly