Add debugging tip to DEVGUIDE.md

[asik]

Added instructions for inspecting optimized assembly, as suggested by dsyme #635 (comment)

[msftclas]

Hi @asik, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. Real humans will now evaluate your PR.

TTYL, MSBOT;

[dsyme]

Actually, after looking through DEVGUIDE.md, I'm wondering if this may be better placed in http://fsharp.github.io/2015/09/29/fsharp-compiler-guide.html. What do you think?

[asik]

Yeah, I thought it seemed somewhat unrelated to the rest... will close this and create a PR on that page instead.