Add rust-toolstate shields to README.md

[Xanewok]

This uses awesome dynamic shields from https://shields.io and queries https://raw.githubusercontent.com/rust-lang-nursery/rust-toolstate/master/_data/latest.json tracked by our awesome rust-toolstate bot.

Rendered

The queries are currently $[2].windows and $[2].linux (jsonpath) respectively, because the data is an array of objects and RLS tool currently happens to be the 3rd one on the list.

@kennytm hopefully this info in README.md shouldn't be useful for too long (because we might want to block nightlies again on RLS fail etc.) but do you think it might be a good idea to change the data format to an object with tool keys { "rls": {...}, "clippy": {...}, ... } to facilitate the quering (e.g. $.rls.linux) or do you think it's not worth the hassle?

Doesn't seem we can dynamically set the color via the dynamic JSON badge, so I hardcoded the color to grey.

@nrc @jonathandturner @alexheretic
do you think this might be useful?

[alexheretic]

Cool! Maybe we should link straight to https://rust-lang-nursery.github.io/rust-toolstate/ instead of the github project?

[kennytm]

@Xanewok Interesting! Since you mentioned this depends on jsonpath, the following expression should work?

$[?(@.tool=="rls")].windows

https://img.shields.io/badge/dynamic/json.svg?label=rls-preview%20(Windows)&url=https%3A%2F%2Fraw.githubusercontent.com%2Frust-lang-nursery%2Frust-toolstate%2Fmaster%2F_data%2Flatest.json&query=%24%5B%3F(%40.tool%3D%3D%22rls%22)%5D.windows&colorB=lightgrey

[Xanewok]

Definitely! Thank you, pushed commits with respective changes.

[Xanewok]

Seems to be working, now the shields are test-pass thanks to rust-lang/rust#53758 (comment)! 🎉

[nrc]

This is so cool; great idea!