[Doc] Add feature branch policy (#2432)

### What this PR does / why we need it?

This patch add the feature branch policy.

After this patch: maintainers are allowed to create a feature branch.
Feature branches are used for collaboration and must include an RFC
link, merge plan and mentor info.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

CI passed

- vLLM version: v0.10.0
- vLLM main:
7be5d113d8

Signed-off-by: Yikun Jiang <yikunkero@gmail.com>

.github/ISSUE_TEMPLATE/750-RFC.yml

@@ -40,7 +40,7 @@ body:
   attributes:
     label: Any Other Things.
     description: >
-      Any other things you would like to mention.
+      Any other things you would like to mention, such as feature branch request.
   validations:
     required: false
 - type: markdown