Beautify arduino-mk-vars.md and add real life makefile example

Hi there!

I took the initiative to *beautify* `arduino-mk-vars.md` by adding code blocks for the examples.
I also added `OBJDIR` as a new variable, because it can be really useful to specify a custom build directory.

I also thought it would be great to have a *real life* makefile example, so I added one of mine. I know there are plenty of great example in `example/` but they are really basic. Here, it shows that you can even more customize the Makefile to suit your needs. And it proves the robustness of the projet.

I really hope it will help! :) If I need to modify/suppress anything, feel free to tell me!

Cheers,
Ladislas