Create Diagnostics ICE-breaker group

[spastorino]

Following the checklist stated on #207 ...

Check list of things to do:

• Create a Github label O-windows (it exists)
• Configure the rust-lang/rust repository to support the ICE-breakers commands, similar to this
• Create the diagnostics toml file in team repo
• Create a sample PR for adding yourself, similar to add nikomatsakis to LLVM ICE-breakers team#140
• Link to the sample PR in this section of the rustc-guide
• Write an announcement blog post

[apiraino]

closing since related to #309