triagebot: Add documentation for transfer command.

Author: ehuss

src/SUMMARY.md

@@ -20,6 +20,7 @@
     - [Documentation Updates](./triagebot/doc-updates.md)
     - [GitHub Releases](./triagebot/github-releases.md)
     - [Glacier](./triagebot/glacier.md)
+    - [Issue Transfer](./triagebot/transfer.md)
     - [Labeling](./triagebot/labeling.md)
     - [Major Changes](./triagebot/major-changes.md)
     - [Mentions](./triagebot/mentions.md)

src/triagebot/transfer.md

@@ -0,0 +1,36 @@
+# Issue Transfer
+
+The `transfer` command allows you to transfer a GitHub issue from one repository to another.
+
+## Usage
+
+To transfer an issue to another repository, enter a comment with the form:
+
+`@rustbot transfer` *repository-name*
+
+It is recommended to also include a comment explaining why you are transferring. For example:
+
+```
+Transferring to rust-lang/cargo since this is an issue with how cargo
+implements diagnostic reports.
+
+@rustbot transfer cargo
+```
+
+**IMPORTANT: There will be no visual indication that the issue is being transferred.** Due to GitHub API limitations, you will not see any activity. **You must reload the page** to view the issue in its new location. It may take a few moments for GitHub to transfer all the data.
+
+**WARNING:** Transferring is a partially destructive command. For example, labels and milestones that don't exist in the target repository will be removed.
+
+The transfer command is limited to team members of the rust-lang org, and transfers can only happen to repositories in the rust-lang org.
+
+## Configuration
+
+The source repository must have an empty `transfer` table to enable transfers *from* that repository. Issues can be transferred to any repository in the rust-lang org.
+
+```toml
+[transfer]
+```
+
+## Implementation
+
+See [`parser/src/command/transfer.rs`](https://github.com/rust-lang/triagebot/blob/HEAD/parser/src/command/transfer.rs) and [`src/handlers/transfer.rs`](https://github.com/rust-lang/triagebot/blob/HEAD/src/handlers/transfer.rs).