add section that provides links to known cross-platform issues

[bcoe]

it might be smart to add a section that links to write-ups on known cross-platform problems.

Thanks for the suggestion @jamestalmage.

[sindresorhus]

Good idea! Some from the top of my head:

• child_process.spawn ignores PATHEXT on Windows nodejs/node-v0.x-archive#2318 (win-spawn and execa in the list fixes this)
• Windows cmd.exe not supporting Unicode (hence the figures module in the list)
• Process exit flush issue on Node.js 0.10 on Windows. (@jamestalmage can expand on this)
• Native Node.js addons are very painful for Windows users.

[bcoe]

a start to this section landed in #5.