Add trimming guidance for developers

[JamesNK]

Introduce devs to trimming. Links to general documentation and ASP.NET Core specific content.

[ShreyasJejurkar]

Just for the information, how many assemblies from aspnetcore are yet to trim?

[JamesNK]

MANY

About 10 15 are trimmed out of maybe 130?

[ShreyasJejurkar]

ok. Thanks. I will take a look at one of them and will see how can I contribute to it! :)

[JamesNK]

FYI @eerhardt @agocke (Just a heads up if you're curious. Let me know if you're not interested in being pinged on trimming PRs)

[ShreyasJejurkar]

Any plans to enable Trim analyzers to catch trim warnings at build time itself.

https://docs.microsoft.com/en-gb/dotnet/core/deploying/trimming/prepare-libraries-for-trimming#set-enabletrimanalyzer

[JamesNK]

Maybe. My experience is build analysis doesn't provide many errors. Publishing with ILLinker is the best way to get a complete set of warnings.

[ShreyasJejurkar]

meaning is there any discrepancy between the errors that you get from Roslyn analyzers and when you do publish!?

[JamesNK]

Yes. It's mentioned in the doc. Not enough information is available at build to do a complete analysis.

[ShreyasJejurkar]

oh ok.

[agocke]

The analyzer should be significantly more powerful in the latest 7.0 previews. There are still some cases that can only be caught with the linker, but as long as your transitive references are annotated, you should get quite a lot through the analyzer.