Update README.md (istio#980)

Automatic merge from submit-queue

Add release-note instruction.

**What this PR does / why we need it**:

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
none
```

Add instruction about how to get collect release-note from PR description.

Should merge after istio/test-infra#441

Former-commit-id: 8c8b8ff

release/README.md

@@ -204,3 +204,19 @@ and edit the release that points to ```${RELEASE_TAG}```. Uploads the artifacts
 
 Create a PR, where you increment ```istio.RELEASE``` for the next
 release and you update ```istio/downloadIstio.sh``` to point to ```${RELEASE_TAG}```
+
+### Generate release-note
+
+First make sure you finished tagging.
+
+This tool helps you to collect release-note left in PR descriptions.
+
+If you want to get this kind of release-note from 0.2.4 to 0.2.6, run the following command:
+```Bash
+$ git clone https://github.com/istio/test-infra
+$ cd test-infra
+$ bazel build //toolbox/release_note_collector:release_note_collector
+$ bazel bazel-bin/toolbox/release_note_collector/release_note_collector --previous_release 0.2.4 --current_release 0.2.6 --repos istio,mixer,pilot --pr_link
+$ cat release-note
+```
+