Document the use of `*` as a placeholder and deprecate the use of `?`

[smarter]

In #91 * was introduced as an alternative placeholder syntax, as part of a long-term plan for repurposing ? for wildcards in Scala (see http://dotty.epfl.ch/docs/reference/changed-features/wildcards.html). To allow cross-compilation, we recently added -Ykind-projector to Dotty to support the * placeholder. However, the documentation of kind-projector does not mention this new syntax, and the old one isn't deprecated, so people are unlikely to start using it. Could this be fixed ?

/cc @milessabin

[LukaJCB]

I can go through cats and change all of the use cases there soonish.

[milessabin]

I'll PR a doc change ASAP.

[dwijnand]

And a runtime deprecation warning, right? (Ideally logging only once per compilation unit, rather for each instance.)

[gabro]

I would also suggest a scalafix rule that could be applied by scala steward once the old syntax gets removed

[VladKopanev]

I have added an issue for IDEA scala pluggin to support new placeholder : https://youtrack.jetbrains.net/issue/SCL-15756

[milessabin]

PR with a simple switch from ? to * in the docs and examples here: #109.

There ought to be a more explanatory paragraph in the README, but I wasn't sure quite how to structure it: putting it right at the beginning is a bit disruptive for someone who just wants to learn about the plugin. OTOH there isn't really a particularly natural break to insert it later, and tacking it on right at the end makes it a bit hard to discover.

[sugakandrey]

@VladKopanev thanks for notification, I've added support for *-syntax to scala-plugin. It is only available in nightly builds right now, but it will be available in 2019.2 release builds as well.
IntelliJ users can migrate to new syntax via Run inspection by Name action, choosing Deprecated kind-projector syntax inspection and then applying Replace with `*`-syntax quickfix on the whole project in batch. .

[jrduncans]

Any progress here? Kind of awkward now that IntelliJ has shipped with a warning that ? is deprecated, but there's nothing on the kind-projector site about it.