Initial documentation for 1.2.0

[CruzBishop]

At the moment, it only mentions the fix to parallel codegen.

I was going to add more, but I have to go for a while - If this isn't reviewed when I get back, I'll remove it and resubmit it after filling in more :)

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[brson]

@bors r+

Nice, thanks!

I have a big patch for this started as well. Better get it in before I get release-note-sniped.

[bors]

📌 Commit 1aabbd0 has been approved by brson

[bors]

⌛️ Testing commit 1aabbd0 with merge 04daae7...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt