Warn when PR is opened for 0.3/0.1 branch

[taiki-e]

This would help avoid problems like #2801 (comment).

rust-lang/rust already has a mechanism for this (rust-lang/rust#118607 (comment)), but I don't know how to enable it for this repo.

[edef1c]

I think having explicit mention of it in CONTRIBUTING.md (or even any documentation about the branching/backporting policy at all) would be a good start. I explicitly looked for some, didn't find any, found that HEAD pointed at 0.4, and figured I should target my bugfix to the release branch.

In other words, it wasn't exactly an accident: it was my genuine best guess at how to get a bugfix merged into a previous release line in the absence of documentation, given that I was uncertain that a fix to master would get cherry-picked to a release branch.

[taiki-e]

cc @Kobzol: Are there any options for doing this in rust-lang/team or somewhere?
I was going to refer to what rust-lang/rust does in rust-lang/team, but rust.toml doesn't seem to exist in the dierctry that futures-rs.toml exist, so I'm not sure what to do.

[Kobzol]

Not in rust-lang/team, this is a functionality of rustbot/triagebot. Also, rust-lang/rust isn't managed by team, it will probably be the last repo to be added due to its complexity 😆

You need to configure this in the triagebot file.

[taiki-e]

You need to configure this in the triagebot file.

Thanks! I opened a PR to enable it.