Add an explanation of the labels

CONTRIBUTING.md

@@ -31,6 +31,49 @@ open target/doc/index.html
 ```
 
 
+## Issue Triage
+
+Sometimes, an issue will stay open, even though the bug has been fixed. And
+sometimes, the original bug may go stale because something has changed in the
+meantime.
+
+It can be helpful to go through older bug reports and make sure that they are
+still valid. Load up an older issue, double check that it's still true, and
+leave a comment letting us know if it is or is not. The [least recently
+updated sort][lru] is good for finding issues like this.
+
+Contributors with sufficient permissions on the Rust repo can help by adding
+labels to triage issues:
+
+* Yellow, **A**-prefixed labels state which **area** of the project an issue
+  relates to.
+
+* Magenta, **B**-prefixed labels identify bugs which are **blockers**.
+
+* Light purple, **C**-prefixed labels represent the **category** of an issue.
+
+* Dark purple, **Command**-prefixed labels mean the issue has to do with a
+  specific cargo command.
+
+* Green, **E**-prefixed labels explain the level of **experience** or
+  **effort** necessary to fix the issue.
+
+* Red, **I**-prefixed labels indicate the **importance** of the issue. The
+  [I-nominated][inom] label indicates that an issue has been nominated for
+  prioritizing at the next triage meeting.
+
+* Purple gray, **O**-prefixed labels are the **operating system** or platform
+  that this issue is specific to.
+
+* Orange, **P**-prefixed labels indicate a bug's **priority**. These labels
+  are only assigned during triage meetings, and replace the [I-nominated][inom]
+  label.
+
+If you're looking for somewhere to start, check out the [E-easy][eeasy] tag.
+
+[eeasy]: https://github.com/rust-lang/cargo/issues?q=is%3Aopen+is%3Aissue+label%3AE-easy
+[lru]: https://github.com/rust-lang/cargo/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-asc
+
 ## Getting help
 
 If you need some pointers about Cargo's internals, feel free to ask questions