Add notes of prioritization labels to doc

Author: giraffate

CONTRIBUTING.md

@@ -310,10 +310,19 @@ currently. Between writing new lints, fixing issues, reviewing pull requests and
 responding to issues there may not always be enough time to stay on top of it
 all.
 
-Our highest priority is fixing [crashes][l-crash] and [bugs][l-bug]. We don't
+Our highest priority is fixing [crashes][l-crash] and [bugs][l-bug], for example
+an ICE in a popular crate that many other crates depend on. We don't
 want Clippy to crash on your code and we want it to be as reliable as the
 suggestions from Rust compiler errors.
 
+We have prioritization labels and sync-blocker label like below.
+- [P-low][p-low]: Requires attention (fix/response/evaluation) by a team member but isn't urgent.
+- [P-medium][p-medium]: Should be addressed by a team member until the next sync.
+- [P-high][p-high]: Should be immediately addressed and will require a out-of-cycle sync or a backport.
+- [L-sync-blocker][l-sync-blocker]: An issue that "blocks" a sync. 
+Or rather: before the sync this should be addressed,
+e.g. by removing a lint again, so it doesn't hit beta/stable.
+
 ## Bors and Homu
 
 We use a bot powered by [Homu][homu] to help automate testing and landing of pull
@@ -327,6 +336,10 @@ commands [here][homu_instructions].
 [triage]: https://forge.rust-lang.org/release/triage-procedure.html
 [l-crash]: https://github.com/rust-lang/rust-clippy/labels/L-crash
 [l-bug]: https://github.com/rust-lang/rust-clippy/labels/L-bug
+[p-low]: https://github.com/rust-lang/rust-clippy/labels/P-low
+[p-medium]: https://github.com/rust-lang/rust-clippy/labels/P-medium
+[p-high]: https://github.com/rust-lang/rust-clippy/labels/P-high
+[l-sync-blocker]: https://github.com/rust-lang/rust-clippy/labels/L-sync-blocker
 [homu]: https://github.com/rust-lang/homu
 [homu_instructions]: https://bors.rust-lang.org/
 [homu_queue]: https://bors.rust-lang.org/queue/clippy