Roadmap for available components

Nobody has claimed this yet.

**Is your feature request related to a problem? Please describe.**

Could we have some sort of roadmap for components so we've a rough idea when they may be available?
At the moment, the official docs at https://material.io/components/ seem to have 3 states for components:

* Not planned
* Planned
* Available

Some components are also marked as BETA, but that doesn't seem to affect whether they will be worked on (e.g. Backdrop [Not planned] vs Navigation Rail [Planned]).

At the moment, it's unclear:

* which components are available in a stable release (e.g. Time Picker is marked as "Available", even though it is in an alpha release rather than stable)
* when we can expect "Planned" components to become available — e.g. there's an open ticket requesting info on the [Banner](https://github.com/material-components/material-components-android/issues/74) component which is marked as planned but there's no details on when this may become available.

**Describe the solution you'd like**

A single place that developers can go to find:

* what components are vailable in the current stable release
* what components are upcoming in the next stable release
* what the planned rough date (or even version number) for components marked as "Planned"

This could be the material.io website, or this repo (e.g. as a README). Ideally, I would appreciate an outline of expected status, something like:

### Latest stable release: 1.2.1

#### Available components

* App bars bottom
* App bars top
* Bottom navigation
* Buttons
* Buttons (Floating action button)
* Cards
* Chips
* Date pickers
* Dialogs
* Menus
* Navigation drawer
* Selection controls
* Sheets (bottom)
* Sliders
* Snackbars
* Tabs
* Text fields
* Time pickers

### Next release: 1.3.0

* State: Alpha
* Expected release date: November 2020
* Components included:
* Time pickers

### Roadmap

| Component | Planned For | Expected Date |
|----------|:-------------:|------:|
| Backdrop `BETA` | — | — |
| Banners | 1.4.0 | Q4 2020 |
| Data tables | 1.5.0 | Q2 2021 |
| Dividers | 1.5.0 | Q2 2021 |
| Image Lists | 1.5.0 | Q2 2021 |
| Lists | 1.5.0 | Q2 2021 |
| Navigation rail `BETA` | 1.6.0 | Q4 2021 |
| Progress indicators | 1.4.0 | Q4 2020 |
| Sheets (side) | 1.7.0 | Q2 2022 |
| Tooltips | 1.7.0 | Q2 2022 |

**Describe alternatives you've considered**
* Make it clearer on the official docs website
* which components aren't ready for production even if marked as available (e.g. adding a tag that mentions "Available (`ALPHA`)"
* what dates components are expected to become available

**Additional context**

The aim of this request isn't to complain that the components aren't being released fast enough (or overlooking the fact that people can contribute PRs to this repo if they have the bandwidth), just to try and make more information available to give developers more context to make decisions against and IMO make the project more usable

At the moment, having something marked "Planned" doesn't really communicate much information to developers.

I definitely appreciate that roadmaps are an estimate against a work-in-progress and are subject to change.

Start with the official material.io components page and the repository README, which the issue identifies as possible roadmap locations. Compare how component availability and release status are currently represented, then document stable, upcoming, and planned components with the release information requested; done means developers have one maintained place to find that status.

Written by the indexing model from the issue text.