Refactor "PDB distributions" section of README

[MattKotsenas]

Refactor the "PDB distributions" section of the README so that:

• Add a small pros / cons table explaining the benefits and limitations of: .snupkgs, Adding to the main package, and assembly embedding
• There's less bouncing back and forth between text (specifically shrinking the distance between introducing a .snupkg and talking about the limitations)
• Add explanation of embedding and benefits / limitations
• Update "main package" alternative to make it clear that it requires consumers to set CopyDebugSymbolFilesFromPackages
• Use GitHub markdown tags for asides or secondary information

Fixes #255

[MattKotsenas]

I originally wrote this change as part of #1270, but separated it into its own PR since this one is a bit more nuanced and contains some opinions instead of just facts.

More specifically, the only downside I see for advocating for embedded pdbs over separate files included in the main package is it's easier to strip separate files for people that really care about size. I'm not convinced that requiring every consumer to opt-in via CopyDebugSymbolFilesFromPackages is a good tradeoff though, and they should instead either publish on a platform that supports snupkg or use another solution like symbol servers.

[MattKotsenas]

@tmat, would you mind taking a look at this when you have a second? If you have any questions or concerns, please let me know. Thanks!

[MattKotsenas]

/cc @tmat, mind making another review? If you have any questions / concerns, please let me know. Thanks!