Warn about use of object type spread

[jnv]

The Flow codebase I work with uses object type spread heavily. Unfortunately this is not (yet) supported by TypeScript, so I don't expect any straightforward conversion here.

Currently Reflow silently drops the spreaded type, e.g. this:

type CustomProps = {|
  foo: string,
|}

type Props = {|
  ...CustomProps,
  bar: string,
|}

is converted to this:

type CustomProps = {
  foo: string,
}

type Props = {
  bar: string,
}

I think Reflow CLI could warn the user about the use of spread syntax. Also the README could mention that the syntax is not supported. What do you think?

[grubersjoe]

Thank you for your feedback! Of course properties should not simply be dropped silently. So I added a warning and adapted the README as you suggested 👍.

Reopen the issue if there are still problems.