Update README.md

description of `FXFormFieldTypeImage` changed to be consistent with other descriptions.
Fixed various bugs in the options and multi-select fields
Placeholder value can now be used to clear options fields
Merge pull request #29 from Ashton-W/patch-1

Update README.md
Better default description for collections
Fixed many fieldDescription bugs
Merge branch 'master' of https://github.com/nicklockwood/FXForms
Cell style can now be override
Button-type cells are now centred by default
Merge remote-tracking branch 'upstream/master'