docs: Add info about ways lead to corruption of container/data

[vitonsky]

How reliable a securefs? How i can broke container encrypted by securefs?

• For example, if i will power off my PC while copying files to container, is my container will broken or not? Why?
• If i will open my container in few devices will modify one file, and then sync changes (for example with DropBox) whats will be? Why?

Add information about it and tell how securefs is resist to this threats.

Maybe securefs is use temporary files while writing and after success writing is write information about new file and then remove old file and information about it and in any unexpected case can be restore all data and remove temporary files? If yes - tell about it.