Add a roadmap document

[kripken]

This topic has come up before, so I wrote a draft of what a roadmap might look like. Does this seem like it could be useful?

[tlively]

The completed work section seems redundant with the changelog and the main readme, so I don't think it's necessary.

It's also unclear to me what the bar for "planned work" is. Is it something our team has committed to doing but we haven't started yet? What about more aspirational planned work? What if someone else plans to do something?

I'm also concerned about staleness in the ongoing work section, but I can see that it would be useful for folks to know what is being worked on. Again, there is a question of what counts. If a contributor not on our team is making sporadic contributions in some area, does that count as ongoing work?

[aheejin]

I don't have strong opinions about the roadmap doc given that the plan part is kind of short, but I agree it'd be nice to have some feature support status. Rather than done/ongoing, it would be nice to have a feature list with ✓/△/X and their phase status as in https://webassembly.org/features/. Of course, we are only one toolchain so the table wouldn't be necessary. Also I don't think we need to dig up Binaryen version numbers where each proposal started. What I meant was mostly something simple like

Phase 5
FeatureA   ✓
FeatureB   ✓
Phase4
FeatureC   △
FeatureD   X
...