notification groups: add information about how to ping them to the respective pages (rust-lang#1818)

Author: RalfJung

src/doc/rustc-dev-guide/src/notification-groups/arm.md

@@ -1,6 +1,7 @@
 # ARM notification group
 
-**Github Label:** [O-ARM]
+**Github Label:** [O-ARM] <br>
+**Ping command:** `@rustbot ping arm`
 
 [O-ARM]: https://github.com/rust-lang/rust/labels/O-ARM
 

src/doc/rustc-dev-guide/src/notification-groups/cleanup-crew.md

@@ -1,6 +1,7 @@
 # Cleanup Crew
 
-**Github Label:** [ICEBreaker-Cleanup-Crew]
+**Github Label:** [ICEBreaker-Cleanup-Crew] <br>
+**Ping command:** `@rustbot ping cleanup-crew`
 
 [ICEBreaker-Cleanup-Crew]: https://github.com/rust-lang/rust/labels/ICEBreaker-Cleanup-Crew
 

src/doc/rustc-dev-guide/src/notification-groups/llvm.md

@@ -1,6 +1,7 @@
 # LLVM Notification group
 
-**Github Label:** [A-LLVM]
+**Github Label:** [A-LLVM] <br>
+**Ping command:** `@rustbot ping llvm`
 
 [A-LLVM]: https://github.com/rust-lang/rust/labels/A-LLVM
 

src/doc/rustc-dev-guide/src/notification-groups/risc-v.md

@@ -1,6 +1,7 @@
 # RISC-V notification group
 
-**Github Label:** [O-riscv]
+**Github Label:** [O-riscv] <br>
+**Ping command:** `@rustbot ping risc-v`
 
 [O-riscv]: https://github.com/rust-lang/rust/labels/O-riscv
 

src/doc/rustc-dev-guide/src/notification-groups/windows.md

@@ -1,6 +1,7 @@
 # Windows notification group
 
-**Github Label:** [O-Windows]
+**Github Label:** [O-Windows] <br>
+**Ping command:** `@rustbot ping windows`
 
 [O-Windows]: https://github.com/rust-lang/rust/labels/O-Windows