update docs

[icebob]

There are some recent changes what we need to update in documentation too.

• change property in schema of file input File upload #172 #173
• supported async validators support async (promised) validators #171
• custom validation message for fields Custom validation for fields #169
• supported string-based validators Provides ability to use strings as validators #167
• vue-multiselect fixed multiselect field error after 'New' #30 but in beta15, but they expanded CSS from JS bundle.

[dflock]

Working on this...

[dflock]

Here is where this is happening: https://www.gitbook.com/book/icebob/vueformgenerator/changes/8

So far, added a validation section and documented the validation stuff - at least as far as I understand it. Will need checking!

[dflock]

OK, I think this is done.

Not really sure about the changes for #173 - I added the value attribute to the attribute compatibility table, because the file input makes the files available there, I think? I also fixed up some breakage in that table.

[icebob]

Nice job! Thank you very much! 🥇