Updated README.md to reflect that FS.File - Objects can't be stored in t...

Updated README.md to reflect that FS.File - Objects can't be stored in the Server MongoDB at the moment.

Hi, I would suggest adding this warning to the documentation, because this bit me in my first steps using CollectionDB and cost me quite a few hours and caused some headache.

The thing is, it's not obvious that storing FS.File-Objects in the server side MongoDB can cause problems while *updating* *if the resulting object becomes bigger because of the update*. That's hard stuff, because at first I wrote a whole form, checking that the insert / creation of objects worked, and after that, when trying to add the "update" functionality to my form, came across seemingly random errors with some form submits while others appeared to work.

Quite confusing and hard to debug, and I wouldn't want anyone to be discouraged from using CollectionFS which is pretty grand otherwise.

So I guess adding a warning would be nice towards anybody new trying to get some foothold.