I agree with the others - a simple, straightforward resignation letter confining yourself to your leaving date, any holiday you have to take etc is best.  

If it makes you feel better, then you can write this person a letter (which of course you won't give her!) about how beastly she is and then tear it up.

It must have been hard to come to the decision you have made, but it's a good one.  You have a good husband and a family and that's all that really matters.  You never know what good things the future holds and being at home with your family is maybe just what you need right now.

I wish you success and happiness always,

Raindance