describe toolstate system

[RalfJung]

Note that I don't know if the rules I am stating here reflect reality. This is based on about a year of empirical observation of what CI is doing. I tried to get someone to make an authoritative statement by filing rust-lang-nursery/rust-toolstate#3, but that didn't work, so now I am trying the alternative strategy of just saying something and relying on people to correct me if what I say is wrong. ;)

In particular, I am not sure if it is true that all "toolstates" are ignored for releasing nightlies.

Cc @kennytm @pietroalbini

Fixes rust-lang-nursery/rust-toolstate#3. Fixes #235.

[ehuss]

This looks great! I would maybe add a section on how to add a tool, since that has been a point of confusion in the past. Maybe something like this:

## Adding a tool

To add a new tool to be tracked, the following steps must be taken:

1. Create a PR to rust-lang/rust that adds the submodule along with any
   necessary build system updates. Be careful that the tests properly support
   `--no-fail-fast`.
2. Include changes to [`checktools.sh`]:
    - Build the tool at the top. This is the step that actually generates the
      JSON status for the tool. When `save-toolstates` is set in
      `config.toml`, the rust build system will write a JSON file with the
      status of each test.
    - Add the tool to `status_check` with whether it should be a beta blocker
      or not.
3. Update [`publish_toolstate.py`] to add the tool. This includes a list of
   people to ping if the tool is broken, and its source repo. (Note: At the
   time of this writing, these users must have permissions to be assignable on
   rust-lang/rust GitHub.)
4. Submit a PR to the [toolstate repo] to manually add the tool to the
   [`_data/latest.json`] file.

[toolstate repo]: https://github.com/rust-lang-nursery/rust-toolstate/
[`_data/latest.json`]: https://github.com/rust-lang-nursery/rust-toolstate/blob/master/_data/latest.json

(And make the checktools/publish_toolstate links into markdown reference links.)

[RalfJung]

@ehuss added that section, thanks!

[RalfJung]

Rebased and added link from infrastructure page.

[pietroalbini]

Thanks!