Simple documentation about using flex in a plugin

[pierrejoye]

Hello,

This is a small doc about using flex in a plugin. Some points may fit as well in the main flex documentation (I may do it later if desired).

I listed most of the points I went through while trying flex in a plugin.

My apologies if I did not follow some styles or standard, please let me know and I will update the PR accordingly :)

[rhukster]

thanks for your contribution.